Since you’re on Cape Cod, some of the trees that are great for Mark in Montana will not grow well for you. I think @scottfsmith’s reports like this one might do you some good. He’s in Maryland. Also, @mamuang, @bobvance, @mrsg47 and @alan are in the northeast and have experience with apple trees. Alan makes a living managing small orchards.
If you need scionwood from a particular cultivar and no one here is able to send you, Fedco sells some.