How about a book? Rent one from the library? I usually stick with John Lyons.
What most people don't understand it that every good trainer should be doing NH. They might not call it that but the concepts could carry through. NH is a commercialized term. I personally don't even look at people claiming to be NH, I look for trainers I've been refered to by friends or ones that share the same principles as I do. Every trainer should be focusing on the horse, "listening" to them and making the training schedule/demands customized to each horse. Training is quite the opposite of Natural. Its not natural for a horse to be ridden at all. So find a trainer, observe them at work, maybe on someone elses horse, or their own. Take a few sessions with them and see if they help your horse (improvements), and if you like them go for it. Check local tack shops, ask local barns... most good trainers are found via word of mouth, from happy clients.