I've found this extended battery on Mugen site. Milestone used to be my phone, and I wish I'd found this earlier.
I'm using Mugen on my Nexus S and Moto Defy and I can vouch for this company.
On Nexus S which is my primary phone I'm up from 8 hours with 3900mAh battery to around 38-45 depending on usage.
So here's the Milestone battery
3300mah extended battery for Milestone