I'd suggest a 2S (7.4v) lipo in the 2000mah range for your nitros. You need a 6v or 4.8v regulator (based on servo specs).

Your other choice would be to use a 2S A123 6.6v battery which is larger and heavier but no regulator would be needed and they are much more stable and safe compared to lipos.

Another option is the use of Li-ion receiver batteries w/ regulator from either Fromeco or Duralite.