The Encyclopedia of Saints by 
The Oxford Dictionary of Saints by At school, your search will take you directly to the article in the database. At home, you will need the username and password which are located on the Community - Online Resources.
Britannica is a great source for biographies because under the "Related" tab, it provides links to primary sources and ebooks. It also provides links to the "Web's Best Sites."
Encyclopedias and databases are excellent sources for biographies. Remember, you may have to consider the different ways your saint is known.