Once you have the ROMs and emulators downloaded, you can actually play retro games on macOS. So don’t worry about finding macOS-compatible ROMs, since such a thing doesn’t exist. As far as the ROM is concerned, the emulator is the operating system.
Note that ROMs themselves don’t care about your operating system. A number of torrent trackers include ROM downloads for a variety of systems, typically packaging hundreds of games together in one torrent.
However, ROMs can still be found in the same places you might pirate other copyrighted content.
