What battery are you using now?
How heavy is your heli with batteries?
Have you looked at getting Lithium Polymer batteries?
These offer the highest energy density (most flight time for the least weight).
If your heli is not grossly over weight, a 2 cell pack should be fine. If for some reason it is heavy (scale body or something like that) then you might need a 3 cell pack.
Kokan 1020, Etec 1200 or something along those lines. Oh, you would need a LiPo charger.