Umbran said:
Even if there are 200+ suits of the stuff around - it isn't magical, it doesn't resize to fit the new owner. This isn't a modern pair of blue jeans you can buy off the rack and expect it to fit, you know. The number of suits available that will fit should be much, much smaller.
There is nothing in the rules about armour sizes beyond Small, Medium, Large, and so forth. A Medium character should have no difficulty finding a suit of armour to fit in a human town.
If the DM wishes to restrict that, he should be doing so equally for armour of any other special material, or even mundane full plate itself. The knock-on effects of doing this are fairly wide-ranging. Unless you're going out of your way to hamper the player just because he wants to get mithral armour.
(Quick question: if the PCs find a suit of non-magical armour on a defeated opponent, do you require that they check whether it will fit any PC? If so, what is the chance?)
Finally, mithral full plate +1 costs 12,500 gold pieces. Consequently, there should be 300 suits of this armour available in the typical small city, all of which
would resize to suit a new wearer.
Plus, what kind of attention is the character bringing to himself as he scours the town, asking people if they're willing to sell their armor to him? Because you won't get me to believe those 200 suits are hanging out in armor shops. They're too valuable and time-consuming to make for that. Those suits are in use, and you won't get it anonymously as ou would a pair of boots.
The DMG states that that is the number of suits that are available for sale. Given the frequency that PCs sell the armour that they're using, I would expect that the armour is probably not in use. Perhaps display suits, suits sold by desperate nobles, newly-forged items made by local armourers, suits brought by travelling elven merchants, or whatever.
If all else fails, the PCs could simply commission an armourer to make the item for them. It will take time, but it should definately be possible in a game that sticks close to the default availability of items.
In any event, there's nothing in the rules or the nature of this item to support making it unusually difficult to acquire. Since we're discussing how this item impacts on the balance of various classes, the core assumptions of the game should be used. And, per those assumptions, there's nothing to prevent a character getting mithral full plate with relative ease.