I finally got around to getting a 18" rear for my 08 WRX. I used a Tusk Impact rim, 18 x 2.15, black. I ordered it online picking the rim for a YZ450F on the hunch that it would mate with an OEM WRR hub and spokes. I does in fact fit perfectly and was very easy to lace and true. For rim locks I went with the Motion Pro LiteLoc, with this I only needed two large lead spoke weights. Originally I tried a standard aluminum rim lock but this needed quite a bit more weight.
I went with the Tusk Impact after reading a positive review in Motocross Action Magazine. It seems to be holding up fine for me.
-Wes