As an avid Flight Simmer, I have found this company to be a great source of the most unusual and out of the way airports.
http://www.navigraph.com/www/default.asp The cost is relatively small, and you'll also find on their site, a regular FMS data update service.
You could of course, also buy the actual charts published by the NOAA through their website.
I have no connection with the above company apart from being a satisfied customer.