Generally, NOAA charts cover US territorial waters, NGA and British Admiralty charts cover foreign waters.  Captain's also stocks charts from the hydrogrpahic agencies of Canada, France (South Pacific French possessions), New Zealand, Australia, and Fiji for their respective territorial waters. The British Admiralty, and the US NGA agency may also cover these waters, but not as completely as do these respective nations themselves.
There is a link from our web site to a NOAA site operated by the US Government where you can view the regional chart indexes, and can also call up an actual image of the chart you are considering to be sure it is the right one.  From the navigation bar at the left of the home page, ckick "marine charts", then at the upper left corner of the next page, under the US flag click for NOAA and NGA Chart Catalogs.  The first four items on the next page is links to order the four NOAA regional printed paper catalogs from Captain's.  At the bottom of that page is links to NOAA catalogs and chart images on line, and to NGA (formerly DMA) catalogs and chart images on line. There is no longer an NGA printed paper chart catalog, but pdf images of the NGA catalog on a CD can be ordered from Captain's