Tractor Supply carries it and many co-op's & feedstores carry it.
Or Valley Vet has a sale on it, right now. Ivermectin Paste (1.87% Ivermectin) Horse Health (Wormers - Paste Wormer - Equine - Ivermectin Hors
When horses are this bad it;s "unofficially" recommended to double-dose again in two weeks which, is why you might want to check with your vet.
I didn't have to do that with mt Arab, I double-dosed him once and he was good for two years (this spring). I double-dosed him again in April or May and he's been fine. I cringe everytime I do this to the horses and continue to be astounded by the results.
Also, if your horse is on sweet feed, that stuff comes thru the pores in an obnoxious way when a horse sweats. It attracts all kinds of flies and knats.
If you do the double-dose two weeks apart or, even wait four weeks, I would put the horse on some form of probios to keep the digestive tract calm:)
I would start that a few days in advance and just go ahead and finish out the container after you worm the horse.
Again, check with your vet who may see a reason why this particular horse shouldn't be wormed like this.
And no, these little microfiliae will never show up in a fecal sample as they don't get in the digestive tract -- they stay in the skin, the muscles and can even migrate into the eye, oftentimes causing moonblindness.
Nasty little things:(