Not all units have challenge coins commissioned and the ones that do exist can be bought on Ebay or through mail order houses by anybody with the money.
A quick search on eBay for "challenge coin" just returned 1588 hits.
What kind of turd would buy one on e-bay and run around perpetrating a fraud like that? Mine was given to me by my Lt. (with my name engraved on it as a gift.) I still carry it 20 years later.