They came on a wide variety of trucks. A lot of people call them "Edge" bed rail caps since a lot of the Edge trim trucks came with them, however I've seen them on 4x2 and 4x4 XLT's.
They were only available 2001-2002, and maybe 2003.
As far as where to find them, you just have to search around. Occasionally they pop up on one of the multiple Ranger forums. Also check your local Craigslist. A buddy found the first set I had on Craigslist, and I just recently bought another set from Craigslist to replace the first set that I sold.