Unfortunately, you're not going to find too many options for aluminum frames beyond Nashbar, Performance and Bikeisland. I know that a lot of people get caught up on components, but really the frame makes the biggest difference in how a bike rides so it is worth investing more into. If I were after an aluminum frame, I would have Rock Lobster build me one. Paul is a well respected builder and his frames are very reasonably priced for custom. Some of the other few builders that still work with aluminum are Spooky and Gaulzetti, though they are way out of your budget.