The original Dexcool coolant had a bad habit of decomposing and eating the caps as well as making sludge in the radiators. Under $10 at Rock Auto on line, just one source, not the only source, and no offense to those members who work for or operate other sources and sneak off from "other duties as assigned" to prowl the site.

 

Pretty sure the one I found was from Amazon or Ebay. Garden variety, nothing special. I did notice that on the old cap I was able to push on the underside and compress it fairly easily. The new one is much more rigid.