I just ordered the Gorilla Gadgets 3500 mha battery for the Samsung Epic 4g (they also offer many others as well such as HTC)
Cell Phones and Tablets :: Samsung :: Samsung Epic 4G - LED Flashlights, Cell Phone Gadgets, Computer Gadgets, iPhone 4 Accessories, and much more with $1.99 Flat Rate Shipping!
Here are some pics from their site.... I cant wait to review it... Currently i use either Midnight rom or Android Creative Syn 1.0.1 rom (currently run)
I hope this is as good as Seidio online extended battery