I bought a set last year, 22mm I seem to recall, these were chrome but decided to spray them silver but as Rockits says they're a pain to take off which finished them off on my first wheel removal but might help with Rockits idea of a drop of grease on them.
If the reason you didn't want to paint yours was because your thought it would be long winded you could always do what I did, I bought a tin of silver paint(in my case it was Hycote Hyundai Sleek Silver) and a small artist brush, sprayed some paint in the spray paint tin lid and painted the ends of the bolts in situ which worked well, do it quite quick before the spray starts going off in the lid because you will end up with brush marks if it's too dry. Take under 5 minutes to do all 20 bolts.
This week I'm replacing my wheel centre caps and I won't be putting my old ones on ebay,lol
Mark
My short lived wheel bolt covers.