Probably an eGo or Tornado or Riva battery (all basically the same thing).
This assumes your map tank has a 510 thread, which seems likely.
You'll need a charger for the battery as well. Most people have 4 or 5 of those batteries (or more), so you don't run out.
You will maybe get a better result with a big-battery 3.7 volt mod but that is getting a bit technical.