I bought a used mountain bike and it does not have a caliper or a brake lever for the front. How can I go about finding out what will work on this bike and where is a good place to buy online?
The disc rotor size is 160mm as engraved on the rotor (160mm is a common size rotor).
The mount type for the disc brake calliper is Post Mount (which should be 74 mm spacing between the mounts, center-to-center), which is the most common standard for mountain bikes.
You can purchase a single Post Mount calliper and matching lever, separately or as a set. My understanding is that 160mm is the standard size to be used without an adapter, meaning only larger rotors would require an adapter. The selected calliper for purchase should have a specification that confirms it is compatible with 160mm rotors.