The Presiden'ts Hundred is now handled by the CMP http://www.odcmp.com/
As far as the firearms, most can be shot with a production firearm (the 1911 is a good example).
The best thing to do is to attend one and instead of shooting on the first match, offer your services. Target pasters, refreshments, score keeper are some jobs that get tedious and having relief is welcome. You will have a chance to talk to a lot of good people and learn some things about their sport.