Jagwire is a good brand as are Shimano's own cables for both shift and brakes. Campy and Shimano take slightly different cable ends for their shifters but the Shimano type is far more common. Jagwire makes shift cables with a fitting on each end, one Campy (embossed "C") and the other Shimano (embossed "S"), and you just cut off the unneeded one. None of these are expensive but be sure to get stainless steel, not galvanized wire.
I've had good experiences ordering from Bike Tools Etc. (
www.biketoolsetc.com), The Third Hand/Loose Screws (
www.thethirdhand.com), Jenson USA and, of course, Nashbar and Performance. My one experience with Niagara was also fine.