REI says there is no recipe to get melon or pumpkin seeds.
So there is no way in a normal Skyblock world to get them.

Wandering Traders provide access to melon and pumpkin seeds, though it could take a long time to actually get them. https://minecraft.fandom.com/wiki/Trading#Wandering_trader

I wouldn't consider wandering traders a viable alternative to FEN. They are too rare and random

Making a release for it soon.