Batteries for mods are the most important investment you will make for your setup. They are not the item to skimp on.
Batteries are not created equal. There are those that are safe to use in mods, and there are those that are not. The link that Katya posted explains this clearly. The following batteries are the only batteries that you should use in a mod requiring 18650 batteries:
AW IMR
18650 2000mah 10A
18650 1600mah 24A
Panasonic or Orbtronic hybrid
CGR18650CH (IMR-Li Ion hybrid) 2250mAh 10A
NCR18650PD (high drain-high capacity) 2900mAh 10A
NCR18650B (high drain-high capacity) 3400mAh 10A
Samsung hybrid (LiNiCoMnP)
INR18650-20R 2000mah 20A
MNKE IMR
IMR18650 1500mah 20A
Sony IMR
us18650v3 IMR 2250mAh 10A
EFEST IMR
18650 2000mah 10A
You will also need a quality battery charger. Name brands like Xtar and Nitecore are recommended.
If you can not afford the above, perhaps you can't afford to use a mod. Perhaps consider a different e-cig setup. I totally understand being on a limited budget, believe me, but don't jeopardize your safety using inferior batteries.
http://www.rtdvapor.com/