My favorite US online store for mods, tanks and batteries is myfreedomsmokes.com. Great prices and fast shipping. Sometimes the pricing is comparable to Fasttech + a couple dollars. Example: my Alien mod Fasttech $49 Myfreedomsmokes $53 and free shipping. Took 3 days to get to me compared to 3 weeks to 3 months from Fasttech.
Runner up to that would be MyVaporStore, but they tend to be a little higher priced.
Chinese sites I've had good luck with Fasttech - Great prices free shipping but slow as Christmas. GearBest - decent prices shipping's hit and miss. 3fvape - Good prices, shipping again hit and miss. Be warned too twice they have sent me a tank/RDA listed as authentic and was a cheap knock off. Banggood - Pricing is hit and miss and shipping can be too. Sometimes they have REALLY great deals to watch for though.