From what I've seen and heard, the cheapest/easiest way to get a decently-accurate measurement is, as Pantlegz1 said, find someone who's well-practiced with calipers. If you live near a university and/or attend school, see about getting measured by a master's student in the kinesiology department. I remember back when I took a weightlifting class in college, their students would usually have already done a few hundred readings, and had to continue doing them for as long as they were enrolled as part of their degree program.