Have you tried Itch.io? They have a lot of Gameboy and NES roms that are “Name your own price.” I don’t have any recommendations - I’ve just started poking around with homebrew roms myself.

• https://itch.io/games/top-rated/downloadable/free/tag-nes-rom
• https://itch.io/games/top-rated/downloadable/free/tag-gameboy-rom