One of the most popular online e-cigarette retailers is Mt Baker Vapor. They sell the usual products that most do, but what sets them apart if their e-liquid. They provide some of the cheapest, but quality e-liquid on the internet.
They have many flavors and if you buy the big 235ml bottles, then you can vape all day for under $1 per day. One of those big bottles can last you a month or two depending on how much you vape.
A downside of Mt. Baker Vapor e-liquids is that you really need to let them steep first before using them. This means to let it sit for several weeks before using to develop the flavor. Baker Vapor differs from many e-liquid retailers in that they make their liquid the same day they send it to you which means it doesn't have time to steep on its own.
My favorite Mt. Baker Vapor flavors: