Where can I get a KORG RK-100?

I got one from ikeytar.net.

Although they're certainly few and far between, they do come up on ebay occasionally, as well as Craigslist. You may find it worthwhile to contact guitar and keyboard stores that sell used instruments, as well as pawn shops, and let them know you're looking for one; they may have valuable contacts or leads that the internet doesn't share.