Check Ebay for "Paint Pressure Pot" ones made by Binks or DeVillbiss are rated to 100-150 PSI above atmosphere (it will be cast into the cover). This will let you test to the equivalent of 150-180 feet.
In my town, I have water pressure at 60 psi, which is equivalent to 120 feet, if it is going to leak it should by then. So, I don't need to add any air pressure. Just fill the tank, put in the item to be tested (put in a dive computer if you want to document the test), put the lid on, attach the garden hose, and turn on the water.