www.stringsbymail.com - strings, guitar accessories and sheet music. 

www.gspguitar.com - a huge archive of guitar related sheet music (doesn't have good descriptions so call and ask quesitons).

www.luthiermusic.com - a great classical guitar store in Manhattan ranging from introductory models to very high end.

www.guitarcenter.com - a big generic music store, on route 46 in Wayne, NJ.  Decent introductory selection of classical guitars. 

www.connorguitars.com - Great guitar maker for those looking for a high quality instrument.  Be prepared for a waiting list.