If the $79.99 list price is to be believed, then Amazon.com has an incredible deal going on right now for a 3500mAh extended battery (with back cover) for the HTC EVO 4G.
Branded by a company called Acase, the battery is currently on sale for just $25.95, an amazing 68% off the original price. The battery is eligible for free Super Saver Shipping too, which is one of the reasons I personally love Amazon (I can't tell you how many retailers treat Hawaii like it's not a part of the US), and is in stock and sold through CTCStore.
How the actual battery capacity compares to the stated capacity is unknown, but $25 isn't much of a gamble to find out for yourself. More photos below.
Acase 3500mAh battery @ Amazon] Thanks, Marlin!