I have a pressage boot but i got it from my vet when my old horse injured her hock and and the wound healed with proud flesh.
Not a very helpful answer from me but perhaps call your vet and see if you can buy 1 from them, they aren't cheap though, i think mine was about £50 and that was nearly yrs ago!!!
But... I ordered an H4 (the largest size) for my 17.1hh and it was too big, so swapped it for an H3 (medium size), but that still didnt stay in place if he moved round much even though it fitted snugly, so I couldn't use it in the field or overnight... only when he was standing in during the day.... he is a big fidget though!!