M116, recently developed, good disease resistance, harder to propagate, not readly available.
Can propagate rootstock by hardwood cuttings.
Triploid apple trees require specific diploid pollinating partners “You will need either one self-fertile apple variety (or crab-apple) or two other varieties which can cross-pollinate each other as well as the triploid variety.” Quote from Triploid apple varieties.
Crab-apple in a hedge if you don’t have room. Can graft crab-apple on suitable rootstock.
M116 relatively new rootstock, better disease resistance, harder to come by.
P22 rootstock, very dwarf. Can use as a test rootstock, for trying out different varieties and also for creating scions. Also, can use for crab-apple pollinator if limited space and want to grow triploid apple tree.
“The only way to do this is to start”! Fantastic advice from Steve :)