Because it's never too early to start thinking about October, here are some websites to get you started on your search!

National directories:
http://www.hauntedhouse.com
http://www.hauntedhouses.net
http://www.hauntedhouseonline.com

Regional directories:
http://hauntedillinois.com/ - Illinois-centric
http://hauntedwisconsin.com/ - Wisconsin-centric

..Have more to add? Please do!

Other sites worth noting:
http://www.rottingfleshradio.com - weekly haunted attraction industry podcast
http://www.ruemorgueradio.com/ - weekly horror industry podcast (movie reviews and interviews)
http://hauntedradio.50webs.com/ - all-horror internet radio show
http://www.hauntedreport.com/ - haunted industry blog
http://www.hauntcast.net/ - haunted industry podcast
http://www.bloodydisgusting.com - horror headlines, movie reviews, and more

Please note: Not all of these links contain safe-for-work images, though the home pages should be ok.