As I like to source my own ROMs, those 4 tend to be my primary places to look:

• https://www.gog.com/forum/general/roms_obtainable_on_gog_compendium/page1

• https://itch.io/games/ (can filter by platform-specific tags for the homebrew ROMs)

• https://retropie.org.uk/forum/topic/10918/where-to-legally-acquire-content-to-play-on-retropie/1?lang=en-US

• https://github.com/farmerbb/RED-Project/wiki/

Also other places, but most way smaller and mainly found through the 4 above.