I get mine from Jeffers Livestock. It is called Copasure and comes in 12.5g capsules and I just divide it into thirds. I believe there's 25 capsules in each container.
I really like Jeffers as well. They run a lot of good sales and sometimes free shipping deals.
If you live in an area with lots of cattle, your local feed store might have them, or a vet. (But the vet would probably charge you an arm and a leg and your first born child, if they would even sell them to you for an off-label species and with no exam...unless you have a super good vet that knows you. )