BMS is spotty. If you know what you want, they will build it and ship it, but if anything goes wrong, you are pretty much on your own. So, use them, but with caution. Their specs are sometimes a little off, but usually they will provide you the details if you ask. The best thing for specs is to go to the individual cells page. For instance the bottle batteries are 0.8C with 1C max. They told me this when I asked, and it is on the individual cell page (18650), but it is not too obvious that an 8ah bottle battery is going to struggle putting out 8amps power. That is obviously a problem.

By far the best battery vendors are (in random order):
LiFePo4: Ping battery http://www.pingbattery.com/servlet/StoreFront
Lithium Ion: EM3ev http://em3ev.com/store/ Sells 2C (NCM), 4C (Samsung) and the renowned A123 batteries.

Keep in mind that lifepo is a bit heavier (it needs more cells to create the same voltage); Li-ion is as light as Lipo, but can't be recharged as often as LiFePo4

They aren't the cheapest, but have great customer support and high quality products

If you don't need the support and are a DIY hobbiest, you can use BMS battery or Hobbyking for Lipo.