Since I honestly dont think that you will find many people on here with spare Hummer parts (the classifieds usually grow cobwebs) and not many wrecked Hummers on here or known about, you might want to consider those are parts you will need to get from a dealer.
Personally I recommend parts like that from a slavage yard since as long as they look good and match the age of the other parts.... you are good. Plus salvage yards will try to stand firm on prices but you can negotiate. ALOT.
Of course if it is a mechanical part.... well..... probably looking at the stealerships
This is just one online site I have found for used parts.
http://www.partshotlines.com/parts/h...fog-light.html
It appears to be a search engine website for recyclers. I looked up something close to me and it gave me like 8 results. Didnt seems bad.... Of course understand I have never used them and know nothing about them, all I have done was a search for you ....
So please purchase at your own regards, but either way tell us how things go.
Good Luck