For one car, I got an STI 4 rad from the Japan yahoo auction site. Fit like a glove obviously.
For the other car I got a C&R aftermarket WRX Rad, from Kastles Corners, it also fit fine. Just have to use a WRX upper hose as the return line is in the center top of the rad.
If you are doing any serious racing/speed stuff, don't use the RS rad, its not made for major heat dissipation, (its too thin!). Use it for testing but don't keep it too long.
(just my opinion)