“So now if my horse has roundworms I have to schlepp around on the dark web like a criminal.”
I suspect that Tractor Supply and similar places have it, but will advertise they don’t. So you’d need to go in there, convince them you have a horse or two and they’ll sell it to you. Just speculation, but with a try before you go to the black market only to buy apple-flavored vasoline.
I ordered online. They wouldn't ship, but I was able to pickup in a week. No questions asked. I expect more problems in the future. Have multiple supply sources for human and veterinary forms. Takes a little effort to avoid the clot shot.