Replace your lost or busted valve stem caps on pretty much any tire.

I used PETG. There is a chamfer 3mm up the inside diameter so you can thread the cap on without needing to print threads. 

Note: you may also push the cap onto the valve stem although I recommend that you thread(screw) the cap on. It doesn't need to be lined up or perfectly perpendicular to the threads, it will straighten itself out while installing.