You will need a flash that has SCA adapter capability.......that will provide the small circular plug that plugs into the side of the camera to tell the flash when to shut down the exposure. Metz, as previously stated, and Sunpak also make flash units that can be used with the 503CW. Sunpak has an H2-A adapter that attaches to their flash units with two cords on it. One is the regular flash synch cord that goes to the lens plug and the other plugs into the side of the 503CW body to control exposure. The Sunpak TTl120j is their top model that will work with the 503CW. I believe that make lesser models that will also work with their adapter.
Hope this helps.