What kind? Google is your friend.
http://cgi.ebay.com/BLACK-For-Magpul-iPhone-3Gs-Field-Crystal-Skin-Case_W0QQitemZ150572447373QQcategoryZ20336QQcmdZViewItem
http://cgi.ebay.com/Magpul-Executive-iPhone-4-Case-Flat-Dark-Earth-FDE-/110661673902?pt=PDA_Accessories&hash=item19c3f323ae#ht_1147wt_905