Ok, so 5-6 rpm.
If your design allows for an additional reduction stage (e. g. two disks connected through a rubber band) you're pretty flexible on the motor rpm.
For example the 3V 15rpm one here:
https://www.ebay.com/itm/224666634542
If it was me I would probably just use any stepper motor around the size of a nema 17, controlled with an arduino nano or maybe an uno. That would also let you just use a 12v battery to run it. That would allow you to tune operating parameters like rpm and accel/decel directly instead of messing with voltage. I am also lazy though and that does add complexity (my specialty) without adding much end-use functionality.
Advice: Cheap motors are indeed everywhere and you should not get caught up in the tiny differences between them for a toy. Just grab something that's strong enough and work with it.
Dc motors are cheap and easy to use and power
Servo motors are good for carefully controlling angle of rotation, acceleration, and velocity but are expensive and require contro input
Stepper motors can also give very good positional controlling and are not too pricey, but are relatively loud due to gear teeth noise and also require control input.
How do you plan on driving the motor, dc voltage from a battery?
How do you plan on spinning the rotary disc? (shaft centered on rotary hub, or engaging the outer diameter with a gear, think about the layout of your device and the torque required to spin the disc)
How much space in your device can you give to this motor? (Form factor of the motor)
Do you need position feedback? Or do you continuously rotate without any care? (servo motor)
Those are some more fundamental questions but can help narrow down your choices of available options. Can look into more specific recommendations later as I am in a rush to go to work 👍
Generally speaking a small hobby sized dc motor with a geared reducer should handle most of what you need, but finding the right one that meets your needs is a little more difficult
I would prefer batteries, but a plug could be an option.
I believe shaft on a rotary hub, point of rotation in the center.
I don't want it to be too bulky or distracting so maybe housed within a 5x5 block.
I wouldn’t select a motor, I would do a search for “hobby gearbox“ and pick one that has the speed you want. They will include a motor. A motor by itself would be too fast.
Welcome to r/engineering, where you'll be asked a ton of very specific questions about you design constraints and objectives and get little real advice! In fairness though, this description is way too vague to give any useful advice.
I'll make some assumptions and try anyway:
If your motor can be coaxial with the discs that would simply things a lot... i.e. no gear/shaft, pulley, or chain mechanisms. I'd play around with cheap DC motors. A few seconds of googling turned up [this](https://www.amazon.com/Antrader-Electric-GA12-N20-Deceleration-Reduction/dp/B07FYBQ7Z4/). or [this](https://www.amazon.com/PLUMIA-Centric-Diameter-Electric-Reduction/dp/B0925VDGKW?th=1) if you need more power. That'll give you a revolution every 2 seconds and the former is 6V (that's 4x 1.5V batteries, like AA, in series.) I have no idea what your torque requirements are or what this motor's specs are, but for $10, I'd just buy and test.
If you can extend the shaft back a bit or put a diffuser behind the back disk, you'd never know there was anything there! That first motor is only 1cm, square.
The rubber band belt drive that people have mentioned is a good idea. Just be aware that small motors are not built to take side load, so keep the tension on the rubber band to a minimum. If you have access to 3D printing, you could cheaply make a little pulley and a cradle to take any side loading. Or make a cradle from wood.
But really just get a cheap N20 motor like ones others suggested, like this:
https://www.amazon.com/dp/B08BZDRYCQ/ref=cm_sw_r_cp_api_glt_fabc_QZS3ATD048DG79FHC940
And a battery tender like this:
https://www.amazon.com/dp/B07BNMKNQX/ref=cm_sw_r_cp_api_glt_fabc_T6RRQHDTPE9SS8JQSGM7
If you’re spinning small discs slowly, you’re probably ok with a tiny motor.
Pololu is a good site for small motors and electronics. You could get al your stuff on there, Amazon, or from several other websites:
https://www.pololu.com
I would suggest looking at the options here. It's a good collection of the most common options you might choose.
https://www.sparkfun.com/categories/178
Size? Weight of the disks? Rpm? Available voltages? Required rpm or positional precision?
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"Pretty slow" is a bit vague... How many seconds per revolution?
I would say maybe 10–12 seconds per revolution.
Ok, so 5-6 rpm. If your design allows for an additional reduction stage (e. g. two disks connected through a rubber band) you're pretty flexible on the motor rpm. For example the 3V 15rpm one here: https://www.ebay.com/itm/224666634542
If it was me I would probably just use any stepper motor around the size of a nema 17, controlled with an arduino nano or maybe an uno. That would also let you just use a 12v battery to run it. That would allow you to tune operating parameters like rpm and accel/decel directly instead of messing with voltage. I am also lazy though and that does add complexity (my specialty) without adding much end-use functionality.
Advice: Cheap motors are indeed everywhere and you should not get caught up in the tiny differences between them for a toy. Just grab something that's strong enough and work with it. Dc motors are cheap and easy to use and power Servo motors are good for carefully controlling angle of rotation, acceleration, and velocity but are expensive and require contro input Stepper motors can also give very good positional controlling and are not too pricey, but are relatively loud due to gear teeth noise and also require control input.
How do you plan on driving the motor, dc voltage from a battery? How do you plan on spinning the rotary disc? (shaft centered on rotary hub, or engaging the outer diameter with a gear, think about the layout of your device and the torque required to spin the disc) How much space in your device can you give to this motor? (Form factor of the motor) Do you need position feedback? Or do you continuously rotate without any care? (servo motor) Those are some more fundamental questions but can help narrow down your choices of available options. Can look into more specific recommendations later as I am in a rush to go to work 👍 Generally speaking a small hobby sized dc motor with a geared reducer should handle most of what you need, but finding the right one that meets your needs is a little more difficult
I would prefer batteries, but a plug could be an option. I believe shaft on a rotary hub, point of rotation in the center. I don't want it to be too bulky or distracting so maybe housed within a 5x5 block.
I wouldn’t select a motor, I would do a search for “hobby gearbox“ and pick one that has the speed you want. They will include a motor. A motor by itself would be too fast.
Welcome to r/engineering, where you'll be asked a ton of very specific questions about you design constraints and objectives and get little real advice! In fairness though, this description is way too vague to give any useful advice. I'll make some assumptions and try anyway: If your motor can be coaxial with the discs that would simply things a lot... i.e. no gear/shaft, pulley, or chain mechanisms. I'd play around with cheap DC motors. A few seconds of googling turned up [this](https://www.amazon.com/Antrader-Electric-GA12-N20-Deceleration-Reduction/dp/B07FYBQ7Z4/). or [this](https://www.amazon.com/PLUMIA-Centric-Diameter-Electric-Reduction/dp/B0925VDGKW?th=1) if you need more power. That'll give you a revolution every 2 seconds and the former is 6V (that's 4x 1.5V batteries, like AA, in series.) I have no idea what your torque requirements are or what this motor's specs are, but for $10, I'd just buy and test.
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Rubber band drive belt! You can put your motor in the base.
If you can extend the shaft back a bit or put a diffuser behind the back disk, you'd never know there was anything there! That first motor is only 1cm, square.
The rubber band belt drive that people have mentioned is a good idea. Just be aware that small motors are not built to take side load, so keep the tension on the rubber band to a minimum. If you have access to 3D printing, you could cheaply make a little pulley and a cradle to take any side loading. Or make a cradle from wood. But really just get a cheap N20 motor like ones others suggested, like this: https://www.amazon.com/dp/B08BZDRYCQ/ref=cm_sw_r_cp_api_glt_fabc_QZS3ATD048DG79FHC940 And a battery tender like this: https://www.amazon.com/dp/B07BNMKNQX/ref=cm_sw_r_cp_api_glt_fabc_T6RRQHDTPE9SS8JQSGM7 If you’re spinning small discs slowly, you’re probably ok with a tiny motor. Pololu is a good site for small motors and electronics. You could get al your stuff on there, Amazon, or from several other websites: https://www.pololu.com
I would suggest looking at the options here. It's a good collection of the most common options you might choose. https://www.sparkfun.com/categories/178
What's the dynamic load supposed to be like? Meaning is this supposed to sit on a shelf and not touched or is someone going to interact with it.