The Library of Congress has an excellent searcheable catalogue, or you could go directly to the Archive of Folk Culture. The Morris Dance Discussion List Archive is searcheable online. Unhappily no search engine yet, but opening times and accessibility are covered. The National Library of Scotland has some online catalogues currently available. The New Hampshire Library of Traditional Music and Dance houses a number of archival collections, including the Ralph Page Collection.
Full listings are available.
New York Public Library Dance Collection is one of the world's largest dance resources. The Public Record Office is the UK national archive site and has an online catalogue. The Royal Commission on Historical Manuscripts covers a wealth of manuscript information sources. The Traditional Drama Website has substantial resources, catalogues and links. The Vaughan Williams Memorial Library has no online catalogue but its pages do cover a variety of useful information including opening times.