Any number of vendors will sell you unflavored, but it's absurd to pay vendor prices for it.
When I first wanted to try it, I went to my local B&M who would custom-mix any of their juices (PG/VG ratio and nic level). I asked them for a bottle of unflavored at my usual ratio and strength. The made it, no problem there, but they charged me the same as any other liquid on their menu. I paid it that time. But the next time, I complained to the owner: "
Look, I don't mind paying your regular prices for liquids made to your own proprietary recipes, but it's absurd for me to pay this much for unflavored. We both know that I can buy the three ingredients and mix this stuff myself for next to nothing." He sold me that bottle for half the usual price, but that was still somewhere between 5 and 10 times the cost of making it myself.