@Jergens : there are medieval armories that still exist that are filled with plate armor for the local militia, on top of weapons. Though mostly in the steel producing areas if I recall.
If the steel was available, then it was not to hard to make or mold generic pieces and mix and match your way to a fit, so they could conceivably do so, plus if the world has been stuck in medieval stasis for long enough even a less prosperous area could amass enough sets for everyone. It is high quality steel and custom manufacture that was expensive as hell.
Though it depends on alot of factors, the medieval period was centuries and very varied, and levels of equipment varried as well. But a magical world changes things alot, as the skills could easily affect what gets produced for what costs, plus monsters could affect what lords feel they need to spend on their troops.