alfalfa cubes!! we could not get weight on our TB and my trainer started feeding him these! he loves them and it put so much weight on him.
smartpak supplements! smartgain...or just use the smartsupplement wizard and that will give you recommendations on what supplements your horse should get.
Legends carb care performance (found at southern states) rice bran, alfalfa, flax seed is what I've used on my TB
This article outlines some good ways