If you want used rims, keep an eye out on Kijiji as well as other car forums. I don't check but I'd guess you could find some more aggressive 17s and 18s floating around on both Subaru and Mitsubishi forums from the Impreza/Evo guys.

Your current rims are not bad at all, but you're right that a) they're not going to be very flush with the fenders and b) its not as easy to get a good stretch out of a tire on an 8" rim (if that's part of the look you want).

Getting wider rims and/or something with a smaller (+)ve offset (like, +35mm) will get the rims closer to the fenders.

Here's the first thing google gave me about an offset calculator, which will help you figure out the concept. Link