Soybean meal is the best source. You just have to be sure your horse isn't soy intolerant. Feeds for Light Horses
Where it says (you have to cursor down quite a ways):
Soybean meal (42 to 50 percent protein). Soybean meal is a preferred supplement for horses. It is higher in protein, has a better balance of amino acids and is cheaper in the Midwest than other supplements.
I fed my 13.3H Arab 1/4 cup daily, divided into two feedings.
An equine nutritionist told me if I started to smell a really strong ammonia smell from his urine to cut the amount back because that was his body trying to get rid of the excess protein it no longer needed.
Once the spring grass came up, I was able to cut his soybean meal back to 1/8th cup daily.
He is 25 years old, I rescued him 18+ years ago starving; he has never had a lot of muscle and literally eats more than my three Walking Horses put together.
He also gets 3 lbs of very well soaked timonthy/alfalfa cubes every day because he has four molars missing.
Your feed store might carry soybean meal. Sadly I can only get it in a 50 lb bag and the Arab is the only horse that eats it --- at 1/4 cup a day so it stays in the house to avoid getting buggy
It is cheap, though