Have you tried the dealer??? They sell catback systems for most of the impreza's I thought. The exhaust systems from turbo to N/A are similar but I think there are suttle differances, not sure if they would work or not, I think they would but mounting might be the issue. Sorry I can't be more help, let us know if you find something that works.
umm you can go to boxer4racing.com and they should have a catback exhaust for you, the company is called stromung, they sound pretty good from what i hear. wrx and sti exhausts could work but you have to cut and reweld the flange because they dont line up, also theres about a 3 inche gap between it so you would have to weld another piece so they can reach. i did it for my impreza wagon and it turned out great lol but its up to you