Game emulators (nintendo, snes, genesis, etc) play games, which are "rom" files. Usually, once you install the game emulator it puts a "rom" folder or similar in its directory (search around using a file explorer app). Or sometimes you can put this "rom" folder anywhere, then direct the emulator where this rom folder is.
Simply search for (nintendo, snes, genesis, etc) roms using google or a torrent site; they're all over the net
Here are some *n e s _ r o m s* that you can try out.