I've bought the majority of my large stash of mods, tanks, kits, and coils from these places below (and a few others that have closed unfortunately). I've been pleased with the products received, the customer service, and their sales/coupon codes:
Rocky Mountain Vapor (has had an on-going 30% off coupon code since last Thanksgiving)
Vapes.com
Desert Vapes
Vapor Beast
LiteCig USA
Ecig.com
Health Cabin (B4 their S/H policy changed recently)
EV Cigarettes
Ecig Supply
Dragonfly Ecigs
The Vapor Room
Eight Vape
DFW Vapor
For bits & pieces mainly (screws, o-rings, replacement glass for tanks, prewound Kanthal coils, KFL shorty parts, Geek Vape & UD wire)...that I'm in no hurry for - I've gotten them from FastTech.