Check your local paper, it should have listings of open houses for the upcoming weekend. Also you should be able to search your local Multiple Listing Service Website. I know here in Charlotte, NC you can search for open houses each weekend on the MLS Website.
There are many forums for searching for Open Houses and the traditional way is obviously your local weekend paper. Of course, the internet is the primary and current approach to searching. Many of the big brokerage firms offer the information on their websites. The big real estate websites generally have a link for "Open Houses". If you are working with a REALTOR, they can actually search their local MLS for Open Houses which meets the housing criteria you are looking for. Feel free to try the link below and go from there. I hope this helps!