You can buy Hepatosupport at
www.homevet.com which contains milk thistle and vitamin b's which are important for liver compromised dogs. When you purchase it, give them your dog's weight, and they will tell you how much to give your dog.
If you prefer to give milk thistle alone, you can buy it at any health food store. If you buy liquid, make sure it contains NO alcohol. Or you can buy milk thistle in capsule form where it's dried herb.