A boot knife will work with just about any boot, the problem is fastening it. If you use combat-like boots, you'll probably need a ankle type holster/sheath (That's what I do), & it works pretty good.
As for the height of the boot, I've used it on 3/4" length (About 5-6") to 9" boots.
As far as where to get them; go to any uniform shop that caters to LE/Safety (Police, Fire, & EMTs), & mail carriers. You should find something there. I don't know if they're still available, as I never tried them, but I recall a boot that was made for LE that had a strap system for a knife.
Again, as has been stated already, check out your local laws as to the legalities of it. While some states allow concealed FBs, most don't allow double edged ones.