Almost any RV store will have "Wheel Center Caps" for sale. There are several different types, stainless, plastic, chrome plated, with/without center plug (for accessing the zerk fitting), all of which are available most places.
etrailer has some of the "best prices" but when you add shipping, none of them are "cheap"....
Here's a link to what they have available.
https://www.etrailer.com/s.aspx?qry=...ries_and_Parts
Amazon also has them here:
http://www.amazon.com/s/ref=nb_sb_no...eel+center+cap