All of the ones I have came on alpha pots I ordered from tayda, like silver said. I found these, from smallbear, which appear to be them. There are alternatives too. Barry at guitarpcb sells pot condoms here, which are silicone.
Yeah, I bought a bunch from smallbear when I started building, before I discovered tayda (where you can get the pots half-price and they came with the covers).
I therefore have a bunch that I don't use, if you need a few I can send some your way.