Researching Dell Part Numbers

Had a Dell Latitude C640 Notebook, now only have some spare parts left. Trying to research them using part numbers either on part itself or on packaging. None of the part numbers match anything on Dell's site. How do I find basic info on these part numbers such as: what models each part  will fit, tech specs, overview, etc? Have found some of them only by chance by entering the type of laptop and model number but this produces only a partial list and sometimes I have the exact same part shown but a different part #. These parts are original parts supplied by Dell for this particular cp. Do I need a cross reference chart or something? Any other web site I can try? Thanks...