You can buy a new hot water tank at any good building supply store, such as Home Depot, or you can go to a business that deals strictly with plumbing. Being as your tank is propane, you might have to search around a little, as they are not nearly as common as natural gas tanks. I do not know precisely what the difference is between them, I only know that they are not interchangeable. The tanks aren't hard to replace, but many jurisdictions require you to get a permit. If you aren't comfortable with the idea of doing the installation yourself hire a pro. Also, once completed, have a thorough gas leak check done, and I would also suggest you purchase a carbon monoxide alert, just in case the heater part of the tank is not properly vented to clear all the fumes out of the house.