To add to what @GPC2012 said here. Nicotine River, GremlinDIY and Bull City Vapor are the 3 i use. Gremlin is first on my list, they carry small 5ml sample sizes on a lot of the flavorings and they accept PayPal. If they dont have what im after i check NicotineRiver and then BCV, in that order.
IMO these are the top three vendors out there.