I highly recommend NCSX as well, and I'd also list:

www.thewestexit.net James over there is awesome

www.markinjapan.com He's not around right now, but he can get LOTS of stuff!

and of course my site, Max's Game Corner (cheap self-pimp!): http://www.angelfire.com/rpg2/maxlords/ I've got a small stack of PCE, Saturn, SFC, and PSX imports available...few hundred including multiple copies of some! I like to think I'm reliable.......just wish I had the inventories of some of those guys with better sources! *sniff*