‘Magic Shops’ are rare!  They are found ONLY in cities with large populations (at least 15,000) and unless the city population exceeds 25,000 or there is a magic school present, there will be only one shop in the city.  It is very unusual for a magic shop to have a public presence on a street.  Generally they will be located in what seems to be a private residence or ‘behind’ some other type of shop; herbalist, book seller, etc. They have a limited clientele who generally have the connections or the knowledge to know how to find these shops.

Most Magic Shops only sell basic material components and the common ‘tools of the trade.’  Blank spell books, scroll making supplies, flasks for potions, etc.  You can also find, (in some shops,) finely crafted wands, rods and staves that are suitable for enchantment.  Materials and ingredients for SOME potions, and Fine Grade weapons (usually only daggers) suitable for enchantment.

Identification and Analysis
Some shops are able to, usually, identify a potion or other alchemical substance.  Not all of them are equipped to do this, but those that are charge 150gp per potion or substance, non refundable, and it typically takes an hour to complete the process.  Results ARE NOT guaranteed!

Magic Items
VERY RARELY, perhaps 1 shop in 10, will have very minor magic items for sale.  (DM will provide info as needed.)  Typically these will be a handful of Potions or similar alchemy products and, more often, Elvin Healing substances sourced through local Healers who do not want to deal with the public themselves.

Material Components
At 1st level it is assumed you start with all the components needed to cast the spells you know at least a dozen times.  (12 grasshopper legs, etc…)  At some point the character will need to spend some money to restock.  The cost of Material Components is lumped together as 5gp per Level of the HIGHEST level spell the Magic User can cast.  So a 5th level mage, who can cast 3rd level spells, will spend 15gp (5 x 3) to replenish ALL of his/her material components (except the weird/expensive etc.)  For convenience assume that the supply is sufficient for a month of steady, daily, spell casting at which point the mage must resupply.  (So for a sea voyage or wilderness excursion that may last several months, the MU must stock up.)
Examples of Unusual or expensive items.  Pearls, gem stones, monster parts, pieces of Saints, etc.  These things (and others as determined) WILL require role play!
Some shops may keep a small selection of gems that are commonly used in casting, but most will refer the buyer to a local jeweler (in exchange for a finder fee from the jeweler) who generally has a larger selection of stones.

Buying, Selling, Trading Magic
Most shops know the magic users and alchemists in the area and also have contacts in the Magical Trade.  If PCs are seeking a spell to be cast or to sell or trade an Item, the shop keeper will get in touch with his contacts and pass on the word.  He/she will NEVER give the PCs names, addresses etc.  They will only pass along the PC’s request. 

Typical Inventory

Clothing, Gear and Armor
Although these items, like most of the weapons listed above, can all be purchased at other shops, they are often found in Magic Shops as a convenience and because there is often customization that Mages desire.

Customizations

Colors: Unless otherwise specified and paid for, clothing; cloaks, hats, robes, gambesons etc, come in a solid color.  Black, grey, brown (home spun), dark blue, dark green, light tan, white.
Specialty colors cost 1gp extra.  These colors include red, light blue, light (‘Sherwood’) green and yellow.
Garments made of two colors (Any of the above) costs an extra 3gp.
Garments made of three colors (Any of the above) costs an extra 4gp. With additional colors adding 1gp each in addition to the 4.
Normal buttons and such are of wood or bone.  ‘Trim’ items, like brass, silver or gold buttons, lace, piping, embroidery and such are done to order and will add 3 to 30 gold pieces to the item cost.

Pockets, sheathes, etc:  Adding pockets, inside or out, to robes and such add 10sp per pocket. (There are practical limits as to how many pockets can be added.)
Boot sheathes and wand scabbards cost 15sp, plus the cost of the boots or garment.

A Magic User’s Gambeson

A typical Gambeson designed for daily use and protection of a Mage (and by some wealthy merchants and nobles) will be of heavier, stouter, treated cloth on the outside then a normal gambeson.   T
ypically a canvas type material, trimmed and reinforced with leather at collar, shoulder panels, edges and cuffs. (It is too thick to be worn under other armor) The interior is padded and lined, either with silk or fine linen for the warmer climates or sheepskin for the colder regions. With warm clothing the sheepskin lined version will serve as Winter Wear in all but extreme cold. It will have a built in belt of leather with attached pouches, two on either side, and loops to attach scabbards, purse or other pouches and devices. There is generally a built in pocket in each sleeve able to hold small daggers or similar sized objects (wand, scroll tube, etc) up to 2lbs each sleeve (2 throwing daggers per sleeve pocket.) Two inner pockets able to hold 4lbs each/ Two front ‘cargo’ pockets able to hold 4lbs each, these have button down flaps. 1 large pocket across lower back (under the belt) that can hold up to 10lbs (suitable for a Traveling Spell Book but NOT a Grimoire.) It will have an attached hood and the fabric is water proofed vs rain (not immersion) 
Cost:  12gp (Basic color with no extras)             AC: 7 
Weight (empty pockets) 8lbs                             Weight (full pockets) 29lbs