2 1500 mAh batteries for $28
My topic here.
2 1500 mAh batteries with external charger. Amazon.com: T-Mobile HTC myTouch 4G 2x1500mAh Battery better than OEM + Wall Travel Dock USB Charger also for Merge TMobile GSM BD42100: Cell Phones & Accessories
Charger has a USB port built into the side so you are able to charge the phone as well.
These batteries say for the myTouch 4G... but are compatible with the Thunderbolt. they are an exact fit, and im pretty dang sure are the same battery with a different name slapped on it.
They got me about 2 hours extra and im a pretty heavy user.