I'm in the middle of doing this for an interior door.

Option A is to order a "slab" door. Measure the interior dimensions of your frame, and look for a slab door that matches.

Example: 30x80.

Just know, that slab doors usually require finishing.

• trimming the sides, routing for the hinges, and boring the holes for the door knob. Some doors have a limit to how much you can trim (ex. <1/4")

Option B is to order a "pre-hung" door. This is more involved since it requires you to remove the old casing and jamb, then replace it with your pre-hung door/casing. I haven't done one of these since it's cheaper and easier for me to install a slab door.