You should look into the lipo packs for the micro helicopters. They have them from
120MAH
http://www.hobbypartz.com/77p-minili...-15c-1s1p.html
to
350 mah
http://www.hobbypartz.com/77p-minili...-15c-1s1p.html
These are real cheap so you could buy several even if you had to change them out more often. I dont think you can get more power in a smaller package.