The best resource for finding a corporate opening is to know someone who knows someone who is hiring. Many corporate positions are filled word of mouth.
However, some excellent resources can be found on the web or through head hunters such as Jet Professionals. Try www.pilotpointer.com as they have a list of the most frequently used web-based listings. Seems to me the most active are avcrew.com and climbto350.com.
Any agency that places employment ads should be checked every so often. Even msn.com had a Gulfstream position listed a few weeks ago under their careers section.