radio shack sells one... don't know how awesome it is gonna be considering it is pretty hard to maintain a set tone, as that is what you are measuring with decibels... basically, how much distance a certain tone/note can carry before loosing its integrity.... get a trumpet and play a B flat... after a certain distance, it is no longer a B flat... that distance is your decibel rating...
Good luck fighting the ticket. I actually hope you win, but that is because I hate cops that do that crap instead of things far more important. You exhaust isn't as loud as the fart cannon crap you see a lot of, and those load ass, annoying fuckers don't ever get pulled over.
While you are looking for a tester and what not, you ought to call around to some lawyers and see what they say they can do.... TicketClinic is pretty good, think they are statewide. Never know, might be able to get it thrown out for the cost as a meter off of the simple fact that the cop that wrote the ticket didn't decibel test your car, he just did it by "ear" and in which case, stock srt-4 neons would get tickets all of the time and they are within the limitations of that statute...