Google Books has helpfully digitized Les deux rédactions en vers du Moniage Guillaume (2 vols., Société des Anciens Textes Français, 1906 and 1911), which is otherwise out of print.

The French Wikisource contains the text of Guillaume d’Orange, le marquis au court nez, a good (if idiosyncratic) 1867 translation of much of the William cycle including the Moniage Guillaume.

The Internet Archive provides a digital version of the Acta Sanctorum; I have excerpted the Vita of St. William, which may be found here [PDF, 1.9 MB].