go to your local farm store: TSC
They have it in the animal medicine cabinet.
There are warning labels on the door, but they still let you buy it.
TSC has various brands of Ivr as a horse paste. I have used the Durvet and Bimectin brands. Been taking it on Sat for the past 10 weeks. Nothing bad has happened. I set the lock ring at the nearest 250 lb increment and push the plunger to inject the paste into my mouth. Doesn’t taste too bad. Wash it down with a few swallows of water.
Forget the pourable stuff. That’s for animals with coat problems. Stick with the paste.
I am not a doctor. This is based solely on my personal experience.