GBA roms are Warez, as its an active console (and Nintendo's current source of revenue)
Sega Genises and SNES roms are abandonware now, so you'll find them pretty easily.
Just go to google and search for the game you want, as there are sites that host direct links to them, as Nintendo could care less about such old games (just as long as no one makes money off of them).
FYI, any warez requests or posting of warez on UGN will result in your thread being closed/a strike being put on you.
Edit: Holy Shit you're fast JC