I wanted to find out what it does for horses so I got some help from bodybuilders.
This link has some good information.
http://www.steroid.com/Clenbuterol.php
It can be bought from an interesting company AG-Guys.com.
I thought it was used for allergies in horses but it has other performance enhancing effects.
It seems to me by doing just a little research any trainer could get any thing they want to improve the performance of horses with out even using a vet.