Haven't tried them yet but I got a supposingly OEM battery (1500 maH) for the Ozone/Imagio at around $10 including shipping via eBay, and I switched it over to the Eris since the Ozone is now used only as an unlocked spare for GSM/wifi purposes, etc. Got it from these folks, fast ship as they're local & was in our mailbox in 4 or 5 days.
It's only slightly larger than the OEM 1300 maH rated battery, but this one fit perfectly as well and with the custom Froyo, rooted phone, giving me enough juice for a full 18-30 hours day without running low - depending on usage, and, at night, I just hook it up to the charger. Plus, I got plenty of vechile & usb charging option while on the go, plus, the spare battery resulting from the switch with me most of the time, so I'm good going anywhere with the device.
I prefer not to throw more money for a 3500 maH battery+cover as none of the protective cover/shield/cases out there are designed for the thicker phone & its bulk, which sort of defeat the purpose of using this relatively "compact" device especially on a side-by-side with the TB and their related siblings.