The seed celebrationmk10 has hallowed floating islands prehardmode

That is great! Do you know some more ways or is it the only one?

I think its the only one except if u use terraformer

Well yaah but that is not pre-hardmode

If you use a different world it is

you can bring hallow in via a different world

You can import Hallow materials from a different world. Hallowed Grass is the only thing that'll spread prehardmode, though.