Vegetable glycerin (VG) or propylene glycol (PG) are the bases in every
eliquid, flavouring, nicotine and usually a drop or two of distilled water and sometimes vodka or other alcohol (especially for DIY people) are added to it. So VG or PG is as plain as you're going to get. I've never vaped this so I can't say what it tastes like, but a lot of people do for something very plain, to vape at night with no nicotine or to clean the taste out of an atomizer to switch flavours.
I think both can be bought at pharmacies and .. I believe it's vegetable glycerin that people are buying at health food stores.
Edit: As Hitmetwice said though, these are both without any nicotine. If you're looking for nicotine you'll have to
buy PG or VG with nicotine from a DIY
eliquid supplier or mix the 2 yourself.