There are three place I would look...
is a "search engine" for junkyards so you may be able to find the part you want at a nearby yard without having to drive all over town looking for it.
Second, RockAuto Auto Parts
has good prices on new aftermarket parts.
Finally, GM Parts Direct: Your direct source for Genuine GM Parts
has good prices on new GM parts. They even have a visual catalog to help you identify the correct part. Be careful on their shipping prices... sometimes they have great prices on the part and outrageous shipping prices. If the shipping is too expensive, take the part number and search www.amazon.com
(yes, amazon) and you will probably be able to find it cheaper.
Hope that helps,