I believe most shops are getting it through Jack @ Thor by the 55 gal. drum.
Some less than reputable shops have been known to cut it -- just be sure you are getting straight goat. If you deal with a shop you trust already, chances are they are selling you the good stuff.
I've got a few gallons here at all times, but I don't buy it by the drum, so our price on it won't be as competitive with some bigger shops.
If you can, buy it locally to save on shipping.
As has already been said above -- get a good understanding on how and when to use goat before you ruin a bunch of good tires. There are times that there's nothing better than goat. The effect it has on the rubber (long term) is enough to keep most guys from using it regularly. Couple that with the carcinogenic and other health risks it presents, and you'll likely use it as a last resort. And hey, did I mention the smell?