I happened to be across the street frm Wheelco Truck & Trailer when I stayed in Sioux Falls, SD. They didn't have my exact replacement jam nuts and covers but had some nice replacements that were a bit taller. The chrome part is plastic but they are tight to the jam nut. The jamb nut itself was a standard part and the chrome covers come separate.
What I did was just buy 4 for my duals since the old (2002) chrome covers were no longer made. I didn't mind that they were slightly taller then the chrome lug nut caps. That way I could point out which were the jam nuts to tire techs. Sometimes they don't have a clue which is which. I've had tire techs try to find a giant socket to remove the chrome cap.
Anyway, here's their web site...and note they have other locations: Wheelco!