Think that with water pressure tanks that around 120 gallons is going to be about as large as you can find easily. You can use more than one tank to gain volume but the drawdown on a 120 gallon tank is roughly a third of the tank size so you would only get 30 to 40 gallons out of one before you lost pressure. A non pressurized tank would give you the full volume of the tank but would require a pump of some sort.