Check with carpet outlets and ask about remnants. Those are the pieces that are left over after someone has their house carpeted. Left over usually translates into cheap. Because the carpet will be cheap, go ahead and get something wild. It's not like you're covering your dream home with the stuff.
Garage sales can be a gem for finding big area rugs for very little money.
Don't be afraid to bargain. Another good bet is to check out bulletin boards around campus. Students who have finished school often sell their old furnishings really cheap to make some quick cash before hitting the road. You can't paint the walls, but covering them is definitely recommended. For goodness sake, make sure you don't use contraband hangers.