Currently the Seidio InnoCell is the only larger capacity battery out for the Evo 4G.
Mostly all of the reviews I have read stated that these battery are on-par if not worse then the OEM Batteries.
HTC Phones Sprint
Best bet is just buy another OEM HTC Battery and call it a day! As everyone uses the phone in a different way battery life will be different for each situation. try some of these battery saving tips for the HTC EVO 4G:
Try to Reduce the Brightness of the Screen ( immense savings)
Go to Settings -> Sound & display -> Brightness.
Try To turn off "always on" mobile data.
Go to Settings -> Wireless & networks -> Mobile networks.
Try to Turn off auto-sync.
Go to Settings -> Accounts & sync -> Auto-sync.
Try to save by turning off the background data. so that applications cannot sync, send, and receive data whenever they want to.
Go to Settings -> Accounts & sync and uncheck the Background data box
Try to Turn off haptic feedback.
Go to Settings -> Language & keyboard -> Touch Input -> Text input -> Vibrate when typing.
Try To Disable Network Notification (Wifi, 4G)
Go to Settings -> Wireless & networks -> Wi-Fi settings. Uncheck the Network notification.
For 4G, go to Settings -> Wireless & networks -> 4G settings.
Try to Use a dark wallpaper. Some say that dark wallpapers are more energy efficient than light wallpapers.
Disable data roaming if you don't need it.
Go to Settings -> Wireless networks -> Mobile networks.
Change the CDMA roaming mode.Go to Settings -> Wireless networks -> Mobile networks.
Tap on "Roaming" and select "Sprint only" from the pop-up window that appears.
These Basics Tips in a way or another will help any Android Phones in the Market (Android 2.0 and up)
How to Increase the HTC EVO 4G's Battery Life