It says on the bag that it's for use in drinking water for swine, calves and poultry, so I bet you can find it at the large animal feed store, too. It's a yellow powder.
It will make the water bottle get moldy, so after about a week, you'll have to bleach the bottle. I just put in a couple ounces of bleach, fill it the rest of the way with hot water and let it set for about 10 minutes - then rinse the hell out of it. I haven't had to scrub the bottle. And don't put it in the dishwasher - unless it's a glass bottle, they don't hold up so well.