Magic items are more than the mundane; they're flowing with inherent magical energy that allows them to perform the functions they're meant to. Although uncommon, magical items aren't completely rare and unheard of, usually being limited in scope to those wealthy enough to afford it - especially in the case of magic items which grant significant benefits.
Not every place characters venture to will have everything they're looking for, or may not have the people necessary to make something as powerful as what they're looking for. This is normally determined by the size and population density of a specific settlement but exceptions of course may exist (for example a hermit mage living out in a hamlet who can make exceptionally rare gear). However, use the following guidelines as a baseline for finding magic items within your area;
Hamlet
Hamlets (or similarly smaller areas) rarely have magic users if ever, and are assumed to lack any sort of magic items unless otherwise specified. If they do however they usually only possess a single magic item, and it likely isn't for sale.
Village
One step up from a hamlet, villages still rarely have even a single magic user however the chances are more likely with usually only a single mage who may also be either the village elder, a lone druid or mage, or a priest running the local church services. Magic items for sale are normally 1d4-1 per month and excess is carried over until the next week.
Town
Towns are more likely to possess mages and are often where individuals meet their first magic user, albeit usually only briefly or from a distance. These magic users sell items on the side in order to fund their personal research, and often are directly controlled by the local government to keep them in check. Magic items for sale are normally 1d6+1 per month and excess is carried over until the next week.
City
Holding specific positions for magic users (known as Magisters), cities have at least a handful in different areas, minding their own business or maintaining their own livelihoods while taking jobs from the government. Magic items for sale are normally 2d8 * 3 per month and excess is carried over into the next month.
Metropolis/Trade Capital
Congregations of magic users all in one specific area create an influx of magic items available to purchase, greatly inflating the value of the region and making living in such areas more expensive than it already is due to being nearby a place with magic items so readily available. These areas normally have any non-unique magic items under 10 Platinum for sale, possess craftsmen willing to take commissions, and there is usually always excess.