The Taming of the Stolen Lands
Special materials like mithral are not impossible to acquire. However, they are subject to the base cost limitations of a city as if they were mundane (i.e., non-magical) items. I also want to note that while mithral armor does count as one category lighter for the purpose of movement and other limitations, it still requires the proficiency of that type of armor (e.g., Mithral Full Plate would require Heavy Armor Proficiency, but it would not interfere with a barbarian’s fast movement or other abilities).
An alchemist’s kit fills the same role for an alchemist that a spell component pouch fills for a wizard. As such, its materials aren’t generally suited for use beyond preparing extracts, mutagens, etc.