I make such things by just taking one of the treasure generators and putting in the parameters I want.
Bear in mind that the "item shop" is more likely to be a broker who knows where various items can be found or made than a commercial establishment in the modern sense. The only "in stock" things in most places are likely to be potions, low-level scrolls, and the odd minor weapon or piece of armour; the rest will have to be made or located. You can still find almost anything you can afford in a big city, but I feel that how you describe the process makes a big difference.