Are you referring to the black plastic screw on caps that hold the wheel center cap on?
If so, you can get them on ebay for sure, maybe Amazon or even the local parts store as aftermarket. I have gotten a couple of sets that way and even found some chrome ones that looked better. The dealer would be crazy expensive on them IIRC.
Those would be aluminum wheel center center/caps and we have heard of the need for them before. There are couple of discussions on here about them, including the likely wheel manufacturer, but I don't know if anyone actually was able to find the centers and get them however. A very tough go with the old Roadtrek out of business and wheels being a probably private label piece from Asia.