For serious students of Classical Guitar, you'll find one of the largest collections of Guitar sheet music scores at www.GuitarPress.com - the URL for their sheet music links page is at http://www.guitarpress.com/sheet.html

While it may not be the prettiest page from a web designer's point of view, it's still a useful resource to quickly (and cheaply) build up a significant digital library of classical guitar works.  There's actually a neat Theory page on the site that includes a chapter by chapter overview of the basics of Western Music History - short, easy to read chapters that read like Music History Cliff Notes extra light.