+1 for Elite cells. Check out
www.cheapbatterypacks.com
Just go through the airsoft menus to get to the TM AK47 (not AK47S).
I have the 8.4v (7 cell) 5000mah one, and the stock had to be modded on the inside to fit it. I'd recommend the one size down. They have awesome prices, awesome product and fast cheap shipping!
Edit: Make sure you switch your gun to deans connectors, you can choose the option to have them pre-installed on the battery (female on the battery), and then all you need to do is cut the tamiya off the gun and solder on a male deans connector. If you don't know how get a gundoc or soldering capable person to help you.