I can't help on brands but often it's a front zip on stiffer dressage boots, but you can also get a zip inserted at the front of any boot, certainly the stiffened ones.
yep mainly dressage boots though some are softer than others. Polo boots have a front centre zip.
I have had Konigs, Petries and Celeris
Mountain horse also make some, there are many others if you know your size you can pick them up reasonably second hand.