6v and I think a lot of us would may agree that the DVD batteries are the best way to go. Not so sure on cheapest these days.
I can't remember the run time on mine now but I think it was close to 6 hours for a 9v 5400 mah
As for a plug I'm pretty sure someone here makes them still. I bought one from a person on here long ago that doesn't frequent here much.
I just had the v3 part of the cable made up and matched it to a connector that worked for my power setup.
You can run right from the battery, the v3 will shed the excess volts as heat.
I know some folks have ran the rc NMH batteries and those can be another option.