I saw some 18's on ebay for sale...

eBay Motors: 18" Mustang wheels black cobra bullitt rims 2005 d5 (item 380107590525 end time Mar-07-09 19:21:58 PST)

or

eBay Motors: Mustang wheels 18" black bullitt cobra rims 2005 a8 (item 220372148178 end time Mar-10-09 19:53:16 PDT)