The following are links to the various clubs that sanction/license agility trials in the US. These links should take you to the event listing page for the organization. If a link breaks, use the Contact Webmaster link below to submit a website comment.
Note, if you register (free) with the AKC site, you can save searches (the "manage searches" tab) to expedite frequent searches. For example, you can set up a search that will list all future trials for MD, DE, VA, PA, depending on how far you are willing to travel.