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I haven't actually looked for it myself, but I know people who bought some. It can be bought for your horse as a paste (for de-worming). You simply need to calculate your dose by your weight. If you eat it with other food, it hides the taste. It is purchased on-line and mailed to you. Just tell them that your horse has worms and make sure it is Ivermectin and not another de-wormer that you are ordering.
There is also a pharmacy company in India that ships the human Ivermectin in pill form. That's as much as I know.