I only have direct experience with Purina Enrich, which I used for my mare- but I've heard that Triple Crown 30% and Nutrena Empower are great as well. You can find the Purina and Nutrena RB at Tractor Supply if you have one, as well as just about anything else you might need. Stay away from sweet feeds (usually called ___% feed) though.
Other than that, you likely won't need anything else besides a salt block or loose salt in a pan for them to eat as they feel the need. If they seem to be older or hard keepers and need something extra, I would first go for something like Beet Pulp (cheaper but must be soaked in water) or Rice bran (more expensive but fed in smaller amounts and no prep needed), or even alfalfa pellets. Only feed grain if absolutely necessary. I only feed my gal grain because she is doing a LOT of growing at the moment (two years old) and even with a lot of hay, alfalfa pellets, AND rice bran, she is still ribby at times so I added the grain for the sake of calories. That isn't your typical scenario though.