Are you looking for trees that are not grafted, i.e ones that are growing on their own
roots? Or are you simply after a larger apple tree than the dwarf/semi dwarfing ones you can find? My grandmother had some large 90 year old apples, they were grafted (they were named varieties so had to be) but were huge and still going strong 90 yeas of neglect later.
Apple
root stocks come in many different sizes For full sized trees you are looking for root-stocks called M25 and MM111 or for slightly smaller trees (13ft/4m) MM106. I use MM106 as it's commonly available. You
should be able to find apple varieties grafted to all of these but you might struggle outside of dedicated nurseries.
online can be a good place to buy them and right now is the right time for bare rooted trees (Northern hemisphere)