The last iMac that will support Snow Leopard is the iMac Mid-2011 (iMac12,1). I have heard that the core i7 version won't, but I don't know if that's true; the core i5 version definitely does.
I bought a refurbished iMac of that model specifically so that I could continue to run Snowy on it, since I develop software and need to be able to test and debug in that OS. (I have 10.6, 10.7, 10.8, and the developer preview of 10.9 on this machine, on various external hard drives.)
You can still find these iMacs on places like Smalldog and other used.refurbished Mac sellers.
