There�s an Axiom center cap on eBay that look identical to the ones on the 18s.
It�s very cheep if some one is looking for one. I�m not totally positive it will fit, But I can�t see any difference from the 18in center cap I have on my desk.

http://cgi.ebay.com/ebaymotors/2002-...3A1|240%3A1318