You can compile your own list from a few sources and gather a fairly complete list. Check Realtor.com (not all Realtors go on Realtor.com and ck off Open Houses - they cannot unless they have paid subscription fees). Check the online and print editions of your local newspaper, particularly the classifieds. The MLS that services your area may make this available to consumers on a public site, or a local Realtor may be able to provide open houses noted on the MLS system.
If Trulia is provided with this information we will display it on our property details page as we do on the property listed below.
Customer Service Representative
Aside from Deborah's recommendations, try Craigslist.com (or craigslist.org). Also, I'm signed up with a local agency that automatically emails me all the open houses listed in our local MLS. Check out the websites of your larger real estate offices or techno-savvy small offices to find one YOU like. Each are a little different so I'm signed up for several.
Good luck. The next National Open House will be with the next time change in Spring.