Renting Storage Space is a rare mission that can be found in the Tavern
's Side Room
You are asked to provide storage space, for a price. You are not required to have enough warehouse capacity (just a trading office), though the profit will be smaller if you have to rent additional warehouses to store the goods.
Note that daily cost of renting additional warehouses is 1 per load (or 10), while own warehouses cost around 0,3 per day.