If your talking about the back plate of the computer, the one that came with the mobo should have an opening. It's designed specifically for the board. You should be able to either get a cable from the mfg, Radio Shack, or online.
Actually any old serial cable from a technician's junk bin or 386/468/Pentium 1 computer found at Goodwill will work. You'e looking for a connector on a ribbon cable joined to a 9-pin d-sub male socket mounted on a slot backplate. This screws into one of the openings behind the PCI slots. These things are pretty ubiquitous. The ATX panel is only for items hard-mounted to the motherboard.