Yes, there really is a written set of rules and regulations for the NMLRA - I managed to score a set at The Log Cabin Shop a couple of weeks ago. It was the only one they had on the shelf, so they may be out now.
Go to their web site at
www.nmlra.org and click on the Charter Clubs button on the left side near the bottom. It'll ask you to specify a state and then show you names/addresses of the local affiliated clubs. There's also a button for something called field representatives - local people to contact about the NMLRA. The site also has information about upcoming events.
I recommend starting out with a percussion rifle - flintlocks are great fun and once you've fired one you'll be hooked for sure, but they require some special knowledge beyond the basics of black powder shooting, and can be a bit frustrating at first. Besides, the percussion guns are enough fun for most people by themselves.
The choice of a rifle is a real personal thing. Both Thompson Center and Lyman, plus some of Cabela's house brand rifles, are excellent quality and very good starter gun choices. The T/C Hawken and Lyman Great Plains are very good examples of the plains rifle design and will teach you a lot about the sport without being intimidating.
Traditions makes some good long rifles in the Kentucky/Pennsylvania/Tennessee styles - long barrel, full stock, a little earlier period than the plains rifles. A whole different feel.
Depending on your personal likes those should be enough to get you started on a very enjoyable hooby that can easily become an obsession. You can get into flintlocks, Civil War weapons, Revolutionary War weapons. French and Indian War weapons, percussion revolvers, shotguns, fowlers, the Brown Bess, Springfields, and the ultimate: a custom made rifle. Or, build your own from a kit or even blanks of wood and metal. The possibilities and choices are endless.
Enjoy.