The fact that I'm linking to stuff here is because I have personally had a really solid experience buying from these online vendors. Bear in mind that the vendors below almost all sell equipment, supplies, e-juice, etc. I'm listing what I've bought from them and why, but check them out in their entirety. These are solid companies worth supporting.
My favorite vendors
Mt. Baker Vapor - I buy almost all of my flavoring for DIY e-juice from these guys. I like their flavors. The prices are stupidly reasonable. And every time you spend more than about $20, they thrown in a free bottle of e-juice just to try. I wouldn't call their juices to be anywhere near premium, but many of their flavors are very good all-day vapes for very little money. They also carry a good selection of gear. Their shipping is extremely fast and customer service is top notch.
Vapor Beast - I've bought from these guys a couple of times. Great prices, friendly service and shipping that arrived before it was actually mailed I think. I once bought a defective item from them, and let me tell you, their customer service is as good as it gets. Every company has a few clunker products here and there, but not every company treats their customers like Vapor Beast.
My Freedom Smokes - This is where I buy my nicotine juice and other supplies. Their nic juice isn't the cheapest online, but the quality is excellent. I've ordered bottles, syringes, other supplies, and a handful of "The Flavor's Apprentice" (TFA) flavorings from them too. Super fast shipping as well.
Fast Tech - If you don't know about Fast Tech, then you will. They are insanely cheap. Not sure I'd ever buy any sort of juice or consumable product straight from China, but I've bought some hardware from them that has worked well and you will rarely ever beat the prices. Shipping takes some patience, but it's usually worth the wait. And they have some cool stuff in addition to vaping gear. I bought a little helium neon laser pointer that is insanely bright and fun to play with shooting it through my clouds of vapor.
101Vape - This place runs deals on mods that are unheard of in pricing. They are literally less expensive at times than ordering direct from China. Both of my iTaste MVP's came from this company. Order processing really isn't very fast and it took a couple days longer than normal to arrive, but given the savings, that was hardly anything to complain about.
Kidney Puncher - I've only made one order with these guys because they carry an extensive selection of Aspire BDC products. The order processing took a couple of days, but it arrived perfectly and I'd gladly order from them again.