A good boot sheath is the first thing. I don't like a naked blade in a boot. Once you have a good sheath, the choice becomes pretty wide. I use a sheath that came with a fixed-blade custom purchase years ago. It has a sturdy clip, is thin and well constructed. That knife is in a drawer; the sheath lives in my boots.
And of course any folder with a clip can be a 'boot knife.'
Since most boot knives are fixed blade, the three I prefer for boot carry are my original Tekna Dive Knife, a BK-11 SanMai leather wrapped, or an original HEST fixed blade. I've accumulated lot of boot knives over my riding career but those are the ones I use currently and work best for me.