If you re looking to repair your item, just find any armour npc and repair it.
If you intent to restore your equipments' max duraiblity after numerous fails in fortification, you can find those repair hammers in the Auction under the 'Supplies' section, and then click the last row. You can filter away the orange,green,yellow to find the hammers more easily.
There are 2 type of hammer that help restore either 50% or 100% of your equipment. Using it is easy, right click the hammer in your inventory, it will be shaded, move your mouse over to the equipment you wish to repair and left click it. Done