Raintree nursery carries several columnar apples but they are unfortunately out of stock.
If you can’t find a columnar apple you might try going with just a dwarf apple. A dwarf apple trained to the tall spindle system can be placed at a 3 foot spacing or you might even consider the super spindle system which is done on a 2 foot spacing. This also gives you access to a wide variety of apple cultivars instead of the few cultivars that columnar apples have available.