As far as "stock" looking exhaust; Cobb, Perrin, and Borla do right of the top of my head, at least for the 08+. Are you wanting to keep the dual exhaust going, or do you not mind going single?
As far as compatible parts, if the flange styles are different you can go to an exhaust shop and have them weld a different flange on one piece or the other. Or if one system is full 3" and the other a tapered or 2.5" end theres an adaptor that is sold on rallysportdirect.com that makes them fit together better and easier.
"2007 WRX- PPG'd, 20G- Tim Bailey tuned
Stage 2 2005 WRX