The maximum USB cable length (total, connector to connector) is 5 meters (around 16 feet). You can buy 10 foot USB cables with a female connector on one end and a male on the other, to act as an extension for the existing cable. A 15 foot cable, unless your existing cable is less than 1.4 feet long, would exceed the spec (and probably not work). There are companies that will make a cable to order, but that would cost you a lot more per foot. (Of course, any extension cable that, with the existing cable, would be less than 5 meters, would also work.)