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So, a Knight walks into a Village's Blacksmith ...
... and orders a new set of Full Plate Armour ...
Bzzzt!
Wrong!
Making Armour was a specialised trade, especially something as complex and technological demanding (for the time and place) as Plate Armour ... which was, incidentally so complex and so technologically demanding that it doesn't appear until the very end of the Medieval Period (in the 15th Century, in fact, which is probably best referred to as the Early Modern period).
Village Blacksmiths had neither the skill nor the tools, and almost certainly not the materials, to make anything more complex than agricultural tools ... if they did, they wouldn't have been mere Village Blacksmiths!
So, a Knight walks into a Smithy in a local town and orders a full set of Plate Armour ...
Bzzzt!
Wrong!
Again, Armour making is too complex, especially Plate Armour ... so, let's try again ...
a Knight walks into an Armourer's in a local town and orders a full set of Plate Armour off the rack ...
Bzzzt!
Wrong!
Plate Armour had to be individually fitted, piece by piece, in the early days and, in any case, it was expensive and time consuming to make ... and medieval craftsmen of any sort simply didn't make much of anything on spec. You had to order it and then wait ...
Which brings us to the point of this Update ...
How common was armour?
Not nearly as common as most RPGs would imply.
The records available suggest that, even in a wealthy Town, no more than one third of households would have had any armour at all ... and of that third, two thirds will have Mail, Jacks or Brigandines. Even in the 15th century only one sixth of the total had even Breastplates. Exactly none had Full Plate.
And, how much was available for sale? An amount equivalent to about 10% of that total ... and close to 100% were Jacks, Brigandines or Mail.
A historical perspective
Putting this in perspective, in 1066 the Population of England was ~2-2.5 million. As a result of the Norman Conquest, William I divided this land into 5-6000 Knight's Fees, not all of which were given to Knights or Nobles (a significant chunk were given to the Church, for example).
(The Norman Army at Hastings had 1500-2000 Knights)
Assuming each of those Knights Fees could provide a Knight, Squire and 3 Mounted Men at Arms, that's around 25-30000 total who would have likely had some sort of body armour (actually less, as some of those Fees were held by the Church and provided no soldiers at all and others were not enough to provide more than a poorly Armoured Knight and even more poorly Armoured Squire). Even so, that's a maximum of around 1% of the population.
And England was a notably wealthy country.
How Long to Make Armour?
Partial Plate (i.e. a Plate Breastplate over a full Mail suit, the best available at the very end of the 14th century) ... 60-90 days.
Mail Shirt ... 15-20 days
3/4 Mail ... 20-25 days
Full Mail ... 30-40 days
But none of the above allows for the almost inevitable waiting period ... for the Armourer to slot you into his current schedule. It's incredibly unlikely your Knight will walk off the street and find the Armourer at loose ends and ready to start work immediately.
Summary
So, not only can't you walk into a Smithy and order a set of armour, and you certainly can't buy one off the rack, even in a Town. You're going to have order it and join a queue.
Fortunately, most of the populace don't have any armour either.