In-Game Resources

You can pick up a longsword at any weaponsmith in town. But where do you go for spells, exotic pets, magic rings or flying carpets? And where can you sell a 5000gp gem?

Shops

Unlike computer games, Faerluria doesn't have a "magic shop" in every city. For magic items, you'll want to find the most upscale sellers of similar mundane items (preferably in the richest parts of the biggest cities). High-end armorsmiths and weaponsmiths, for example, may keep some minor magical equipment in stock; seek a cordwainer for magical boots. But unless you're looking for something very simple, like +1 armor, you'll likely have to commission the item you seek. The smith, tailor or whatever will collaborate with a spellcaster to make what you need, if you have the cash and the time.

A few shops carry a random magical stock: Wickhelm's Rare Items and Household Furnishings Emporium, for example, sells magical loot found by the more adventurous Wickhelm relatives. Each empire's capital has a few such places.

Expensive gems, jewelry and objects of art can be bought and sold in the upper district of Caer Cefn, and in the southeast quarter of Dinas Mawr. (Or they may advisedly be kept, if they are less cumbersome than their resale value in gold would be.)

Private collectors

Private collectors (usually greedy creatures such as dragons or nobility) may part with their magical items, for a steep markup or for large favors. You'll need an "in" to negotiate with them at all.

The Mages' Guild

Fees: For spellcasters looking to gain arcane spells, membership costs 100gp times the square of the highest level of spell sought. (Previous fees paid do not count toward upgraded membership fees.) For all others, membership costs 350gp.

Membership entitles you to:

30% discount on copying spells from the Guild library.
The chance that the Guild library has a spell is:
1 - (spell level / 10)
(Exception: 1st level spells are all available and free.)
20% discount on copying spells from other Guild members.
Guild members may post lists of spells that they offer.
The chance that a spell not in the Guild library is posted for offer by a Guild member is:
0.85 - (spell level / 20)
(Note that both of these discounts apply to the fees (50gp/level), not to the material costs (100gp/level).)
Free spell exchange with Guild members.
A Guild member who offers a spell may waive the fee, in exchange for the right to copy one of your spells of identical level. (Material costs must still be paid.) The chance that the member wants a given spell is:
50% if it is not offered by any Guild members.
10% if it is offered by Guild members.
0% if it can be found in either the Guild library or the Great Library at Corstref.

All of the spells in the Guild library are also posted for offer by Guild members. (Exchanging spells with them is cheaper than copying from the Guild library.)

(Often, there is an agreement that both parties are free to trade away the spells that they just received, or that neither is. Otherwise, you get dumb situations in which someone trades everybody's spells to everybody else, making no contribution of their own. Magical enforcement of this agreement, or magical testing of veracity, is impolite if done overtly, but is acceptable if both parties can plausibly pretend that it didn't happen and that they simply trust each other.)

Spell exchange with the Guild library.
Again, the spell levels must be identical. There is no "right to copy" fee, but you must pay for half of the materials for the Guild's copy. (This costs slightly more than just copying a spell; consider the difference to be a donation. If exchanging spells with the guild were profitable, the market for spells would collapse, and/or the guild would go bankrupt.) The Guild will trade for any spell that it does not already have.
Magic item shopping.
The Guild store will buy and sell magical items, at half and full price respectively, and also has a a 30% chance of hooking up a seller and buyer directly, for an equitable sale at 75% of retail price. (For simplicity, you may just average this, and sell all magic items through the guild for 57.5% of list price.)
Discounts at participating stores.
Guild membership is good for a 10% discount on spell components, inks and paper, and a 5% discount on spellbooks and potion ingredients, at any shop that displays the sign of the guild. (These can be found throughout the empire of Tyr Delyth, including such outlying places as Dinas Felin and Corstref.)
Reference library use.
Free for members. You get a +2 circumstance bonus for knowledge (the planes) checks, and +4 for knowledge (arcana) checks, while using the library.

The Great Library at Corstref

Basic membership (500gp) grants access to common books.
+2 circumstance bonus to the following knowledge checks: Architecture and engineering, dungeoneering, geography, nature, religion.
Historian's membership (1000gp) allows access to rare books.
+2 circumstance bonus for knowledge (arcana) and knowlege (the planes). +4 circumstance bonus to all other knowledge checks.
Magician's membership (1500gp) allows access to all books, including unique and magical books.
+4 circumstance bonus on all knowledge checks.
You may copy spells from spellbooks freely (but must still pay your materials cost). The chance of finding a given spell (arcane or divine) is:
0.55 - (spell level / 20)

Membership is for life, and may be upgraded by paying the difference.

Temples

Assuming that you pay your 10% tithe, your temple can arrange for healing and other divine spellcasting services. Your tithe also goes toward particular services or discounts that vary between temples.

Different priesthoods also offer goods and services, depending upon their specialties. For example:

Hollingsworth Specialty Breeders and Ranch

This is where you go for special animal needs. They carry all sorts of creatures for pets, mounts, familiars, hunting companions and more. They provide custom training, and can cure your pet of its nasty habit. And they accept exotic creatures, so you can drop off that dire slug that followed you home after you critted your animal empathy check.

Hoggelrynth School of Magic
(or your private tutor)

Wherever you learned magic (your choice), you have a source of information about magic, including knowledge (arcana) and where to find spells. Give your teacher and his/her resources/contacts the same chances as the Mages' Guild and its membership, but instead of a discount, add 50% to the standard fee. Their time is valuable.

Corstref Academy (or the city guard)

While magic weapons and armor can almost always be sold to the local city guard (for 75% of list price), it has to be something they can use. (e.g., they probably won't want a gnome hooked hammer if they have no gnomes.) The academy in Corstref trains warriors and aristocrats of all races that can afford it, and will buy practically any martial equipment (also for 75% of list price).

Elminster, Fizban and David Copperfield

For most high-level spells and powerful magical items, you're best off making the personal acquaintance of high-level magicians (which is likely enough once you're at the appropriate level).