If you don't mind bittorrent and you have cable internet or don't mind waiting forever, search torrent sites for "GRH.se" and you'll find EVERY SINGLE ROM for various systems. (GRH.se used to be a big ROMs site but now they're a crappy "show off you retro game collection" site)
Also,
www.pleasuredome.org.uk has a complete MAME ROMs torrent