I have this one.

http://www.testersandtools.com/Ampro...k-Detector.php

At the time I bought mine it was $322. You can save money by eliminating the sound generator, but as Ed correctly points out it helps show openings not easily detected. Place the generator on the co-pilot seat and from the outside the leak detector will tell you where the door seal is not making good contact.

These are not the answer for finding every leak, but they are one of the ways to detect leaks.