how do you guys know about any organized centuries around your area? where do you go to look for when centuries are held?
A good national guide to charity rides is: http://www.goodcyclist.org/rides.htm
Bike clubs often have websites and maintain lists of local organized rides, including centuries.
Local sports papers and magazines for triathletes and distance runners will also list organized bike races and long rides. Check for free papers at your LBS.
or google "new york century bike" or something like that, assuming you are looking for NY rides.
This won't do you any good unless you like to travel, but since you mentioned it.
Great reference for California riders.
Love to Ride