Not the scale you"re interested in but I use a lot of horse manure for composting. Horses here are routinely dewormed, takes awhile before the drugs won"t kill soil worms.
The stable nearest me (this is horse country) is thrilled when I show up. They"re more than happy to load me up with 7 tons or so even though their tractor is on the small side. Disposal is a common problem here.
The other risk is they never know what broad leaf herbicide might have been sprayed on the hay they bought. "Long-lived" is no exaggeration when an herbicide is still active after the trip through the horse"s gut. Your use might not matter, I"m growing veggies.
They load, I haul. No money exchanged. Everybody"s happy. Their only question is when am I coming back?