Try to search hecell on ebay. Or true rc. I bought some generic packs on ebay and they were crap. Be careful! Also try cheapbatterypacks.com, their elite cells look good, and are cheap enough! Plus that site has a discount section that has some decent stuff at times.
Well, what gearing and speed are you going for? 40amps at 11.1v is around 400 watts at most, maybe good for 35mph? I would gear conservatively and keep an eye on temps on the first few runs. Then gear up until the batts start to give.
I have run similar motors in my tamiya tt01 and a 4000 11.1v maxamps pack kept up. We all know what i think of their ratings, so I know that Dan's (true rc) packs can keep up. His stuff is underrated.
Amp draw is more with more voltage if you do not regear. The 5700 on 4s is way too much, and at that point, it's just a matter of "when" the motor will break. Probably sooner rather than later too.