I've never seen them sold separately, but then again, I haven't seen everything.
Caps for the STM Links are weird, though. I have the ones that came with the mouthpieces and they are bent in way that makes them odd. If you switch to a non-Link ligature, you may be able to find a cap that will fit . . . and you may be able to find a non-stock cap that will fit the Link ligature, but I don't know where to tell you to go for that. DAVE