From what I've read in other post Wallmart sells it.
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There have been mixed reports on the flavor and purity of the Walmart (and CVS/Walgreens etc.) varieties.
Pretty much everyone I've heard from agrees that the NOW brand 100% VG is top quality and can be found in many health/vitamin stores around the country.
Finally
this source from Amazon is definitely well reviewed by folks in the DIY forum. It's available with free shipping, is 100% pure USP grade, and is about as good as you're going to find price-wise per oz without moving up to 1/2 or full gallon size.