Depending on the quality you want, eBay. On eBay you can find a lot of different bumper style cases and bumpers for the iPhone 4S and iPhone 5.
Other web sites sell varations of bumpers, but they are costly, upwards of $34.95 for one.
People seem to like SGP bumper style cases. Those are not cheap.
Amazon.com has bumpers for the iPhone 4 and 4S and I believe for the iPhone 5 as well.
Apple still sells bumpers for the iPhone 4 and iPhone 4S. They don't have any for the 5.