I don't know where you are located but I have picked them up over the years at auto flea markets. If you happen to live in an area where the cars last for a while, you might pick up a set in a junk yard. They were used from 1974 right up to 1989 on the full size Chevys, especially the cop cars. I had a couple of 86-90 Caprice police cars, both of which came with those caps.

Here is a better picture. My last two cop cars were just like this except one was was ex-RCMP white, and the other was unmarked charcoal gray.