The International Betta Congress
The IBC isn't just your average club. It contains all the information you could ever want about bettas in their TA Library. The IBC coordinates all the shows and events. They can answer all your questions about bettas. Although it is an international group, the IBC is split into districts and chapters; that information is also available on their website.
Website: http://ibcbettas.org/
Website: http://ibcbettas.org/