What is the substitute if you don't have the pot?

You can use a standard stovetop pressure cooker, if you have one large enough for the pan to fit in. If you don't have a pressure cooker at all, you can bake it in the oven- though I'd recommend baking in a waterbath. Finally, you can steam one on the stovetop- in the manner of Japanese style cheesecakes. A quick websearch should get you the oven temperatures and basic directions for these methods. You can choose whichever fits your need best.