Do keep in mind to pay attention to not only the size (18650 or 18350 mostly), brand (pick Panasonic, Sony etc before considering saving a few cents and getting a 'no-brand' (chinese) model. They are all made in China, but Quality Checks do matter a lot) and mAh (anywhere from 1000 to 3400mAh is available for both mentioned sizes), but also to the number indicated with most good batteries which is followed by a 'C'. For example
"Sony Konion US18650VTC4 3.6V 2100mAh (15C)"
This indicates the amount of power it can actually deliver in a certain amount of time, measured in Amps. Higher is better. If you want to go into low-Ohm coils or even sub-Ohm, you should not get anything under 15C, but go for 30C (or Amp) if you can afford the price and the fact that the battery will probably be of lesser mAh (so will need charging more often).
Also, the mentioned mod is completely new on FT.com You might want to stick to the thoroughly reviewed ones that are also available on the same site for the about same price and have pretty much the same looks.
As for batteries, I think FT.com still has an extreme amount of shipping costs on batteries, since they had some trouble with transport by several airlines late last year. You might want to order as close to home as possible for the battery itself.
ps. If you get a mechanical mod WITHOUT any battery protection, make sure to get protected batteries.
