As you travel around on your journey, you will find many different shops, with some very potentially helpful items. But keeping track of which shop has what in stock is somewhat difficult. Below is the location of each shop in Zelda, and what they sell as well as the cost of it for that shop. So not only can you find the item you are looking for, you can locate the best price for it as well.
Once you take the letter from the Old Man, the Medicine Woman will begin selling potions in various places. This is certainly a must do thing, especially later in the game, when dungeons get more brutal. Below you will see the locations of all the different Potion Shops found in Hyrule. All shops are the same price as well. 40 Rupies for the Blue potion, and 68 Rupies for the Red Potion.