Anker™ 2 x 1800mAh Li-ion Batteries for Samsung Galaxy S I9000, GT-I9000 + Free Anker™ Multi-purpose USB Travel Charger $13 +$7 S&H at Amazon:
Amazon.com: Anker™ 2 x 1800mAh Li-ion Batteries for Samsung Galaxy S I9000, GT-I9000 + Free Anker™ Multi-purpose USB Travel Charger: Cell Phones & Accessories
Scroll down to specs. Rated OK for Epic 4G.
1800mAh is 300mAh more than stock.
It's the best deal I could find today.