Like I mentioned, it's not the best way to build a leakdown tester, but it will give you the basic idea. I'm pretty sure you can buy internal PVC pipe plugs that already have a NPT thread hole in them. Then use brass reducer bushings to get down to 1/8" NPT. Get a 0 to 15 PSI gauge like on page 5 of that thread. Grainger is a good source. Also, possibly a welding supply company. For the tire valve fitting, go to an auto parts store and buy a "tank valve". It has 1/8" NPT threads on it and is for use in a portable air tank. For the exhaust, get the rubber caps and hose clamps like on page 5. You can also use the rubber caps off your stock front bumper. To presurize it, get a small basketball/tire pump.