Agree that your regular hay (if good quality) should be fine. I was going to say alfalfa, but it may be overkill, then saw you're feeding the pellets so wouldn't bother.
Do find a good M&F feed. They should at least be willing to order something? Then keep that (you shouldn't need a supplement with that) and the alfalfa. You need to be careful with pregnant/lactating mares, when they drop it can happen overnight. That's why people often keep them a little on the heavy side. I know it's a little late at this point, but you really need to keep ahead of the curb. Worst case you wean the foal early. Really pour the food into her, free choice hay and a LOT of the M&F and pellets. There should be guidelines on the bag, and I would check in with your vet too.
I'd just really push the M&F instead of messing around trying to come up with something else that will put weight and give the right nutrients.