Since a Dutch oven is basically just a deep covered pot, any pot with a lid that is of sufficient size and depth can be substituted. Soup pots or deep skillets may work in a pinch.
It is important to ensure that the alternate pot or skillet is designed for use in the oven if oven cooking is involved in the recipe. If the pot is going in the oven, it is also very helpful for it to have handles on the sides. If the recipe calls for cooking on the stovetop, any heavy pot that has a lid and has the required capacity is sufficient.