Army surplus stores do sell used Black Caddies, but by the time they get there what little ankle support they offer is shot.
If you‘re looking for something to equate to the boot to practice long marches, etc, get a pair of hiking boots with good ankle support so that you don‘t hurt yourself, then prepared to be disappointed when you‘re in.
When you do join, spare no expense buying good insoles. If you are in a health plan that provides for custom-made orthotic insoles, definetely make arrangements to get some.