Horse Health clearly made in USA. Jefferspet and Durvet look to be imported (but recall that the Durvet was previously made in USA as well as other brands). https://www.tractorsupply.com/tsc/search/ivermectin?isIntSrch=written
You may want to consider country of origin in addition to price —for this and other products.
I do not feel uncomfortable with Canadian Manufactured Drugs.
Durvet is made about 20 miles from my house.
DURVET, INC. ANIMAL HEALTH PRODUCTS P.O. Box 279 ยท Blue Springs, Missouri 64014