Every town has a limit to the type of resources that can be found within its borders. Within the D&D framework, this is called the GP limit. The GP limit is the price for the most expensive item that can be purchased in a settlement of a given size. Usually, the larger the town, the higher the limit… To provide a real-world example: you are much more likely to find a Ferrari dealership in Seattle than you are in Forks.

However, while the game provides a rule for the GP limit based on a settlement’s population, I believe this rule should be flexible.

House rule

The GP limit sets the maximum value for items that can be readily found within a settlement of a given size. (i.e. on your shopping trips, you can automatically purchase items from the PHB or DMG that fall below this maximum price¹) However, if you wish to purchase an item whose value lies above the GP limit of a given settlement, you may make a search or gather information check to locate a vendor who carries the item. This check will increase in difficulty the further the item lies outside the GP limit of your location.

An example result of a successful check would be that you discover a reclusive wizard who fled the politics of the arcane guilds in the big city to pursue his studies in peace, or you find a retired soldier who was once a great swordsman and is still in possession of his gear.