Well, not like those nice packaged ones such as wendel's herbs, but I have on occassion been able to buy directly the herbs that were the active ingredient in something that cost an arm and a leg for the pre-made herb mix.
Quite a few years ago when I still lived out west, my mom's horse got terrible founder and ended up with bad navicular. She went through many vets and farriers to help but didn't get very far. Once I moved to her area and saw what was going on, I found her a natural hoof trimmer that knew what to do for him and I did research and found a particular supplement that was really good and vet reccomended, but really costly.
A little more research into the active ingredient, an on line trip to the herb store and I was able to purchase the exact supplement ingredients. Gave them to him and he started getting better and lived quite a few more years.
There are some good books out there on using herbs for horses that were helpful in my search.