I was asked where I have been getting my LiPo batteries so here you go.
http://www.hobbyking.com/
It is shorter and thinner than an 8.4v mini and has a higher RPS rate, a bit faster than a 9.6v NiMH. It fits in the hand guard of M4's and the stocks of large battery guns. This includes the PEQ size battery compartment of my RPK. $7.49 from the US warehouse right now at Hobbyking.com.(Update 09-21-12, out of stock US)
Currently on back order from the US warehouse (the Europe Warehouse has them for $9.84, but you will have to wait an extra week for shipping) the 11.1v 1400mah stick also fits into M4 hand guards as well as large stocks. It will fit inside the buffer tube on a crane stock (I have tested that) but I am uncertain about a front wired AK. At $9.84 its a steal. Exarach has 2 and they shred. (Update 09-21-12, in stock US)
Not as wide, nor as tall, and half the length of a 9.6v AK stick battery these 7.4v 1200mah nun-chuck are perfect for a crane stocks and front wired AK's. (Update. I have tried this battery in the Dboy RK-02. Its about perfect. I will be ordering the 2000mah version when I get my own Dboy AK) They cost $5.75 each and cannot be ordered from the US Warehouse. About 3 weeks for shipping. The 1400mah is $0.99 more.
My future purchases may include 11.1v 1200mah crane batteries for $8.72, if I get that little monster running the way I want it to.
If you truly possess a sense of adventure or just like to fix your broken guns, you can buy a high discharge/large capacity LiPo from their RC section and convert the plug.
Don't forget the multi-charger for $17.99 plus a power supply for another $10 This one is a smart charger that charges and balances up to 6 cell LiPo's and NiMH/Cd's up to 17 cell. Computer, cool. All in all about $35 shipped.
(Update 09-21-12, in stock US) For the all day+ shooter, the 7.4v 2000mah crane battery for $9.68. This battery also fits into Dboy and CYMA AK's.