Any protected 16340 @ 3.7V will work. You'll need a pair of them. I'd suggest 2 sets of pairs so you always have power as the other set charges. Decent ones cost about $5-$6 a pair direct from China and take about a month shipping this time of year as little as a week during less busy times. They can be had for about $10-$12 a pair from a few places in the US. I don't know of anybody with special deals right now. madvapes.com has good prices, here's a link but there are plenty of other places to get batteries also. Don't forget a charger if these are your first batteries.
Li-Ion Batteries
Li-Ion Batteries
