I get mine from a health food store, not particular brand though.
If you want Marin or Denamarin, check here. Both are good and these prices are awesome!! You do not need a script for either supplement.
My holistic Vet sells and swears by Standard Process products. You can find them on the web and sometimes on eBay. Good products, but they can be hard to find locally.
Yes, milk thistle helps the liver.