Answer: If you already have an apple tree, it might be simplest to graft the branch onto the existing tree.
There is a selection of rootstocks available and your local county extension may be able to suggest which one(s) would be best for your planting site in addition to being dwarfing, semi dwarf or standard in terms of the size you need. You may find the following extension publication helpful, Rootstocks for Fruit Trees in Wisconsin
Once you have determined which one to use, they may be able to help you locate a source. Otherwise you might try catalogs such as Raintree
or One Green World
Good luck with your project!
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