Ebay and Amazon.
http://www.amazon.com/gp/offer-listing/B003VFREMS/ref=dp_olp_0?ie=UTF8&redirect=true&qid=1280499727&sr=8-3&condition=all
http://shop.ebay.com/i.html?_nkw=apple+bumpers+authentic&_sacat=0&_dmpt=PDA_Accessories&_odkw=apple+bumpers&_osacat=0&_trksid=p3286.c0.m270.l1313
http://shop.ebay.com/?_from=R40&_trksid=p3907.m570.l1313&_nkw=official+apple+bumper&_sacat=See-All-Categories
Sorry but they're all expensive.