The dealer is the only option for these, and if you buy a complete fog lamp assembly, it is a very small amount more than just buying a lens. And I would buy a PAIR of lenses together. And some Lamin-X to protect them from sandblasting, cracks, etc.
Only other option would be if you find a NOS lamp, or set, on eBay.
Don't bother with used. 99% of the time they are going to be junk.