It is usually possible to dismantle wooden staircases, especially if they have been assembled without glue. The stringers, treads, risers, balusters and handrail can be disassembled and salvaged. However, this requires excellent preparation and coordination. For smaller staircases, it is often easier to recover them in one piece - and this is the form in which they are sometimes found at some dealers.
There are also cast iron or steel internal staircases, both straight and spiral, which are usually sold as individual pieces.