If you want a reliable tree form of Serviceberry, stick with A. arborea, A. laevis, or A. x grandiflora. One of those three shouldn't be too hard to find.

A. canadensis will probably require a little more work to maintain tree form than those other three.

Plants offered as Amelanchier canadensis may often not be that species anyway. In addition to there being a history of misapplication of species names to cultivated stocks of plants in this genus botanists have never completely agreed on how to interpret wild populations of the North American kinds. For additional discussion of the latter situation see:

http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=101333