You can get food grade propylene glycol at The Chemistry Store
for $17.88 per gallon. I just ordered another gallon for my humidor, and I mix it 50:50 with distilled water. A gallon lasts me a really long time for my humidor, and it is far more economical to buy it this way than to buy a 50:50 solution for $10 a pint from my tobacconist. Shipping from SC to TX was $10.61 UPS ground. They charge a $3 handling fee, so my total cost was about $31 for a gallon.
The price is $16.05 a gallon if you buy five gallons. That's probably a more reasonable quantity for a refrigeration system. Or, you can get 55 gallons for a mere $759 ($13.80 a gallon).
If you're going to use it in a refrigeration system, be aware that the food grade product doesn't have any additives to prevent corrosion build up in your plumbing system. Pay a few pennies more and get the version that has corrosion inhibitors added to it. Remember, the glycol runs through a closed system. It isn't going to get in your beer.