Look for a local wood shop as it will not be an off the shelf door from any one. Fontrick, Select, Creekside, Upstate Door will all make one for you as well. You can also look to Tru Stile Doors as they will make it as well.

Try a local salvage place. If you're looking for one inside door to match others, it's reasonably likely that a number of houses in the area had the same doors and that some of them might have ended up at the salvage store.

https://www.rockwooddoor.com/custom-luxury-doors/interior-doors/