One of the more affordable extended battery cases, the New Trent Rock case IMP220B ($34.95) has a 2200mAh battery.
The case is available on Amazon but there are two versions of it. One costs $34.95 and one (which says it's for the 4 and 4S) costs $44.95. It's unclear what the differences are so we've linked below to Newtrent.com, which offers the 4/4S case for $34.95. See pricing for New Trent Rock case IMP220B from NewTrent.com.