Will it be fed as a powder with chaff or sugar beet or are you after more of a complete feed? If a complete feed - then info on the horses and work levels would be needed!
We feed Top Spec Anti Lam balancer to our horses who are all good doers. At first glance it seems expensive but you only feed a very small amount. Lots of people use Baileys Lo-Cal and are very happy with that.
If your horse isn't a good doer there are other higher feed value balancers out there, it depends what you are wanting to achieve. The reason we feed a balancer is because we keep our horses on fairly restricted grazing year round and soak their hay in winter to avoid them putting on the pounds. Feeding a balancer ensures that they get everything they need nutritionally without gaining weight.