I would venture any feed mill ,has a sheep balancer protein pellet. Appropriate amount of that ,mixed with grain, should do nicely. If a feed mill ,is making sheep feed ,they have it.
Supplies all of the vits,minerals, usually has deccox or lactosoid, and has ammonium chloride for urinary acidification.
I would not feed sheep ,with out it. 300 to 500 pounds per ton of grain, and you got a pretty nice grain mix. I would use some oats or barley, to cut the corn,if hot weather is an issue
Late gestation,lactatiin, and breeding, protein is your friend.