FWIW the ‘horse drug’ and the people drug are exactly the same, pure ivermectin. The only difference is that the ‘horse drug’ version is 1.87% and the people version is 1%.
And Amazon still has it available, though it’s almost $20 a tube, rather than the original $6.48 at Tractor Supply.
Durvet horse paste Ivermectin was $4/tube at both Amazon and TSC in November 2020. I know because that’s when I purchased my initial reserves.