For a 6th gen (not 7th gen) iPod nano, the cable is the standard 30-pin to USB docking cable, that Apple previously included with iPods and other devices. Apple still sells it; the official replacement is
http://www.apple.com/shop/product/MA591G/C/apple-30-pin-to-usb-cable
NOTE: If you have a docking cable that you use to sync iPod to your computer running iTunes, it's the same cable.
There are also numerous third-party versions of this cable from the usual online sources, like Amazon. Doing an Internet search on "30-pin USB docking cable" should provide results.
One odd thing to consider is the size of the plug for the external powered speakers. I have an older 5th gen iPod nano. The dock connector and headphones jack are right next to each other. If the "handle" around the headphones plug is too large (fat), it does not fit when docking connector is also attached. I don't have a 6th gen nano, but you can check how much space is available between the two connectors.