If you are looking for an LE or SE, these are usually easy to find on Ebay of in the junkyard. But if you are looking for the SSE, you'll have a very tough time finding one that's in good used condition. Last year I sent an email to
www.gmpartsdirect.com and they gave me 4 different part numbers for the SSE taillights: left inner, left outer, right inner and right outer. They are two-piece assemblies that are glued together. All four pieces were over $300 so I was just going to replace the outers and try not to destroy the inners. I haven't ordered anything yet.
I have no idea where the email is so I'd have to dig it up if you need the part numbers. Or you could email them yourself. If you run across a nice used pair of SSE tails, let me know!