I find it dificult to locate pure propylene glycol in feed stores except during very cold weather. Sometimes what is stocked is an uknown dilution.
A more reliable, easily accessed source for most people is an RV supply store. Get drinking water system anti-freeze.. It likely will have something added for color, but I have experienced no problems with it. Cost usually runs about $16 per quart. When I buy a gallon at local chemical supplier it runs about $60.