http://www.modelelectronicscorp.com/products.html
http://www.a123racing.com/
At A123Racing website you can buy either loose cells, and you can buy the pre-assembled packs of which you can get the Turbo versions; which is a variable voltage circuit (very snazzy!) you can adjust the voltage and the voltage will be stable under load (i think it has no Volt droop).
Or you can build your own pack and attach a balancing cable for the chargers they sell.
I however do not use a balance cable (not near as critical as Li-Po), i use a BK Precision lab power supply to charge my 5s2p pack in 25 minutes. It's great, they stay room temp while charging too.
One of the best things about the A123 M1 cells is that they seem to deliver 100% right until they dump, its very abrupt!