Building a frame opening to fit the salvaged door is more advantagous than finding a door to fit an existing aperture. 
Consider different hanging methods; hinged, track sliding or pivot hinging. 

Glazed or half glazed internal doors can help small spaces feel lighter.

Multiple doors can make a cost effective panelling. 

(Above) Mahogany framed doors conceal utility appliances in this Parisian appartment
(Above) A set of doors in their original condition are reinvented as a sliding door that runs across the front of this garden cabin come guest bedroom.