Any vendor that sells parts for 47-54 Chevy trucks would have a cap. You could probably find one in a salvage yard also.
If you're looking fo a locking one, I got mine from The Oldie Goldie Shop, really nice quality, and has a cover over the lock with a chevy bowtie emblem on it. It's nothing like the cheezy generic locking caps you find at department stores. I'm sure other vendors sell the same thing, but that's the only one I know of for sure.
