This is just my experience with buying used parts for my GN so I cant say this happens to everyone BUT, every body part I have ever bought used has had to go straight to the body shop.
You may be able to find a black bumper but if your running a fiberglass then you most likely need the support behind it as well. So your looking at needing a complete bumper which I believe once Brian (Gbody parts) is done moving may have for some amount of money. I dont want to sway you away from buying anything from a sponsor but if you got a connection to a body shop you could get a garbage chrome bumper used complete with support brace for dirt cheap. Dont strip it, just sand the crap out of it to get it rough and then spend a hundred bucks to get it painted.
Just my .02, hope you find what you need.