I have always used this site for airport diagrams. The FAA updates these diagrams every month and they don't cost anything since they are used for reference.http://188.8.131.52/digital_tpp.asp?ver=0508&eff=8-4-2005&end=9-1-2005
You just enter your aiports ID. Ex - DFW, MSP, ... Once you do that, you can get all kinds of information, If you want just the a/p diagram you look for the "Airport Diagram" link and it will open up a PDF worksheet of the entire airport. See picture down below.