AppleJunkie - The list they have up now is just the stuff they KNOW is available right now for their inventory. As this summer progresses and their grafted trees grow, they'll add more to the list later as they determine which ones will be big enough to sell for the spring shipping season. They usually add many more trees on various rootstocks as the summer winds down. I wouldn't worry that MM-111 rootstocks aren't on there now.
You can call Cummins also and get their assessment for what's likely to be available. They always answer calls I've made to them. If you don't get an immediate answer in person, leave a message & they'll call you back.
Just saw Chicken posted much the same info a few minutes ahead of me !! ^^^^^