Its the same OEM battery they sell at Verizon and is distributed by HTC. At Verizon you can still pick one up for 30.00 though via the 4g battery sale (50% off)
I use the extended battery and I get excellent battery life. 2-3 days off the charger. I actually purchased 3, but I only really use 2 of them to switch out to avoid leaving my phone on a charger. I haven't plugged in my phone to charge since I acquired these batteries and the standalone thunderbolt battery charger.
Currently there are 3 cases on the market for the external battery. One I wont even mention since its a horrible hack job.
full TPU case
LINK
Kick stand holster case
LINK
I prefer the TPU case since I like the fit, feel, and the front protection it gives. The holster only provides front protection when in the holster which I don't use.
Plus the TPU case is less bulky.
If you need a standalone battery charger, you can pick one up for 5.00 at Verizon. Just buy the Thunderbolt model. It is compatible with the rezound batteries.