Have you tried searching in ITA?

http://matrix.itasoftware.com

or

http://matrix2.itasoftware.com

in matrix2, you can month long, using a range of stay dates up to 7 days difference (e.g., 2-9 days, 25-32 days, etc.)

You can't book tickets directly through ITA, but the fares shown are usually bookable on other sites.