You can buy a carbon fork with disk tabs and 74 mm dropout on aliexpress for $65, but I'd sooner have tabs welded onto a steel fork. Those adapters are pretty weak. I paid $3 for one and decided it was it really flimsy, and it only fits certain fork geometries. Some people just tack weld them to their forks and add a little reinforcement.

My Downtube (100mm suspension forks) has tabs for disks, but I would have to spoke a hub up to get rotor capability. I've found that in general, adding rotors/calipers to a bike that had good rim brakes is a wash at best. It's only good if you do a lot of wheel removal.