I use this one...
http://www.hobbypartz.com/83p-4000mah-4s1p-148-20c.html
Pretty cheap for the power it puts to the starter. I can start my 40cc Zenoah with it!
Just be cautious...Ive burned starters out on gas engines using a 4s by holding the button too long. Smoked it! lol
But that was my own fault. The engine wouldnt start and I kept on and on and on...then the magic smoke left from the starter?!?!?!?! lol