Look for Glycerin at Walmart, on the shelf above the rubbing alcohol products in the pharmacy section. It's a 6 ounce bottle for $3.00. That is vegetable glycerine or VG. You don't need to add much, though, to get more vapor. Like, up to 20% or less of your total mixture. Some people (like those who made YouTube videos are mixing 50% Glycerin with their juices), but that takes away too much flavor for me.
Edit: Some of the guys in some of YouTube videos are adding 50% Glycerin. I'm back to adding about 33% Glycerin to my pre-mixed liquids, due to having sore gums and sore areas in my mouth, probably from either too much PG, nicotine, or high-voltage heat. I haven't determined what's causing the mouth pain yet, but it isn't due to too much Glycerin.