Pig droppings?
Find a pig farmer, He may even pay you to take it. If movies have taught us anything its Pig**** = Fuel. Fuel = who rule Bartertown?
Now imagine what Trolls, Ogres, or dare I say Dragons could do for you.
You may have to compete with some Otyugh to corner the market. But summon a Gelatinous Cube and "clean up"