The Biography in Context database provides easy access to historical and current people. Articles come from reference books, magazines and newspapers, and websites -- all with citation information readily attached. However, you may notice differences in the types and amounts of coverage for historical and current people.
The Internet Movie Database might be one Internet source for biographies and work history for Seinfeld actors. Try it if you can't find a lot in the Biography in Context database. You might also try searching the actor's name in one of the databases listed under the Articles tab on this page.