Looks like this is the right cable but they don't ship to US.
http://www.amazon.co.uk/USB-voltage-converter-cable-negative/dp/B011LO5KJE%3Fpsc%3D1%26SubscriptionId%3DAKIAJJKQFJJZIZTE4NFA%26tag%3Dantiviexplai-21%26linkCode%3Dxm2%26camp%3D2025%26creative%3D165953%26creativeASIN%3DB011LO5KJE%26ascsubtag%3Ddefaultcustomid
