My $0.02. If anyone has better information, please correct me...
There seems to be about four types of bumper fillers on the market (besides NOS)...
1. ABS Plastic - sold by several Buick vendors. These are harder plastic than the OEM ones, so they won't be nearly as flexible as OEM. I've seen both good and bad opinions on these on this board. I am guessing that all of the vendor's ABS fillers ultimately come from one supplier. These are among the cheapest options.
2. Fiberglass - sold by several Buick vendors, including GBody, the GSCA, Kirbans, and others. These are very rigid and will sometimes require a good deal of prep work to fit properly. I've seen both good and bad opinions of these on this board. I am guessing that all of the vendors' fiberglass fillers ultimately come from one supplier. I originally ordered these for my car when I had it painted, and neither the paint shop nor I liked them. It would have taken a lot of prep work to get them to fit. These are also among the cheapest options.
3. GBody's reproductions - sold by GBody and Kirbans (maybe others?). These are OEM reproductions, so the material, flexibility, and appearance is just like OEM. I have not seen a bad opinion yet on these fillers. I ended-up going with these when my car was painted (after not liking the fiberglass fillers), and both the paint shop and I really like them. They look OEM on my 43,000 mile car. The downside is that they are more expensive than the ABS or fiberglass ones.
4. SpoolFool's fillers - sold only by SpoolFool for now. I have no personal experience with these, but you can go to the links he posted a few posts ago for more information. I have yet to see a negative review about these on this board. SpoolFool's rear filler is one-piece, which is different than the overlapping two-piece OEM design (which GBody emulates). You can see for yourself the cost when you look at the links he provided.
More Information:
http://www.turbobuick.com/forums/threads/goody-parts-vs-kirbans-front-fillers.390630/
or you can do a search for "bumper fillers".
My humble opinion: If OEM look and feel is most important, maybe for a low-mileage show car, than GBody's would be best. If you're not as concerned about OEM appearance but want "the best" fillers, you will do well with either the GBody or SpoolFool fillers. If low cost is of highest importance, maybe for a driver car, then either the fiberglass or ABS fillers will work, just be prepared to do more prep work to get them to fit and take paint.
Hope this helps...