The Ore Seller, run by Ordan, is one of many of the stores in Keldagrim and can found downstairs in the Blast Furnace factory in the eastern side of Keldagrim. This shop stocks ore used for Smithing. The shop is restocked by dwarves that regularly enter the room and deliver a number (around 30) of one type of ore to the shop, rather than restocking one of every ore periodically. When the shop reaches 100 of a particular ore type, the extra are "sent to storage".
An alternative for buying tin, copper, iron, silver, gold ore, and coal is the ore store ran by Hring Hring on Jatiszo. The store has a max stock of 20, 10 or 5 (depending on the ore) and is further from bank but is a lot cheaper and restocks quickly.