TOMNKY...thanks for clearing up that pressure testing is a good way to test for a leak. You said you were getting hot, but you did not say your were losing fluid.
In fairness to you, you asked if we knew where to get a cap that would help you pressurize the system. I do not.
Air goes places easier than water. Look in the radiator while running, and you will know if there is a path to the cylinder from the water jacket by seeing lots of bubbles, because cylinder gasses will pass through any gap faster than water will.
Pull the dipstick and look for water, or change the oil. If there is water, it came from a head problem. If there is not, there is not problem there.
If no oil in the water and no water in the oil, there is not a problem there.
If I HAD to pressure test, and could not find a cap:
If you know the fitting size of the sending unit, it should be easy enough to pressurize the system through that. Your manual should tell you the releaf pressure of the cap, etc. So pressurize to just below that.