Sears used to carry this sort of thing. Home Depot or Lowes has these things for private water systems. You unscrew the pipe that connects it to the pump output, unfasten the supports, and replace it. If the tank is not one made especially for the pump, you may have to do some "engineering" I grew up in a far suburban area in a house that had a private water system. the tank had no bladder, so the air pocket above the water slowly dissolved into the water and every few weeks you would turn the pump off, let the pressure out and open a valve that would let air into the tank. After the tank was empty of water, but full of air, close the valve and turn the pump back on and the tank would fill with water until the pressure switche turned it off. If the tanks has a "bladder" there is a layer of rubber separating the water from the air so that the air won't dissolve into the water. If there is a leak, the air pocket will slowly fill with water over a few weeks and the pump will run excessivley. On some tanks I have seen there is a shrader valve like on a tire at the top and you would use a tire inflator to fill the pocket with air...
