Fiveoboy01, you are limited to 6v hump packs only. Any brand 5 cell hump pack you get will fit in there with alot if room left.
Since most 6v 5C packs comes standard with Futaba J type plug, and the harness on the Arena accepts the BEC type plug you can use this adapter if you don't want to cut and soldier any wires together. Personally, I think it looks alot neater and easier using this adapter.
http://www2.towerhobbies.com/cgi-bin/WTI0001P?&I=LXWV18

I use hump packs made with intellect cells. The price is cheap and they perform excellent.
http://www2.towerhobbies.com/cgi-bin...&I=LXMGW5&P=ML

Same exact pack....
http://www2.towerhobbies.com/cgi-bin...?&I=LXMUE0&P=7

Here is a picture with a Venom 1200mah 6v hump pack I use for practice.


As you can see there is alot of extra room in there. I use some tire foam to pad the battery.