CRB Chapter 6 covers the basics. Generally magical items are for sale wherever the GM deems appropriate. I believe there is a settlement level in the GMG that helps determines what level of magical items are available, but I don't have my copy of the GMG yet so can't speak to that. At the GM's discretion more or less can be available in a given area. This goes for item rarity as well, uncommon magical items can be available at the GM's discretion just as readily as a common item.
The rules for purchasing gear in general are very loose in PF2. You should restrict your players to purchasing gear of their own level or lower as a rule, but that rule can be broken at the GM's discretion just as the rarity restrictions can be lifted.
A good rule of thumb: Villages and small townships probably won't have more than the most basic magic items available. Maybe a few healing potions, some talismans and perhaps a +1 weapon. As the settlement gets larger, the better chance that more skilled craftsman will live there and create stronger items. Capital cities may have hidden magic shops that sell the highest end equipment, but only to those they trust or know.
At the end of the day, you have to strike a balance between letting your party get geared out, and restricting them from items that may break game balance, and sprinkling quest rewards and loot into the mix so your characters don't have to just buy everything they want.
There’s also the rules for rarity to help you. Most items are of “common rarity”, which means they might be for sale in a typical market that sells items of that value. Some are “uncommon” or “rare” or “unique”, which decreases the chance they could be purchased or created.
If you’re thinking of rules along the lines of “how do I know how many level 3 common items should be available at market”, I don’t think there is a specific rule for it - a suggestion would be to have number of players +1 items of the party’s average level for sale, either selected carefully or randomly rolled depending on taste.
Considering it takes a craftsman 4 days with the proper equipment and formulas to make anything he can make then I'm inclined to allow most common items regardless of lvl (if they can find a craftsman of high enough ability) with the caveat that it might take 4 days to get/make it.
Considering it takes a craftsman 4 days with the proper equipment and formulas to make anything he can make then I'm inclined to allow most common items regardless of lvl (if they can find a craftsman of high enough ability) with the caveat that it might take 4 days to get/make it.
Availability of vendors and downtime are totally up to the DM, so even with Craft you might not have the formula for what you want or the time to make it.
To the OP: there is a section in the Gamemastery guide for this. The Dm sets one of three possibilities for availability: "PCs can buy what they want where they want" and "Magical markets are rare or nonexistent." are the two extremes. In the middle is "PCs can buy what they want but must put in additional effort.": "This approach allows PCs to determine some of their items, but forces them to really work to get more powerful items and discourages looting every enemy to sell
off fairly ordinary armor." Pg#24 gamemastery Guide