The best deal on GM parts is probably www.gmpartsdirect.com. The problem is the charge a lot for shipping and handling and sometimes take too long to get the parts out. If you're not in a hurry for the parts, give them a try.
They actually come out cheaper than gmpartsdirect due to the fact that they include shipping in with their prices. It is about 5 - 10 dollars but to me that is money saved. They send you an email receipt with tracking # and the shipping is fairly prompt.