Please make this a sticky. I just remembered a company called AuVeCo. They sell the exact hardware that the OEMs use. Their catalog is a bit of an adventure to find anything, but many of those hard-to-find trim clips are in here:
http://www.auveco.com/OnlineCatalog/index.html
Here are some handy references from the back of the catalog:
http://www.auveco.com/OnlineCatalog/index.html
Here are some handy references from the back of the catalog: