Stuart, I have two Drones here, I also had an armasight battery pack to power them, I sold this on, its just extra weight in my mind and bulky on the scope. I buy the cr123 cells from amazon I think there around £11-12 for 10 of them, I just turn the drone on for the shot then turn it off, to be honest they last me ages like this, and its much more compact, I use the thermal for spotting vehicle mounted and handheld. Do you have a separate spotter to use?
PS if you wanted to use an EPS3 supply you could just make up a little convertor using a Buck regulator to drop it down from 12v to 6v,