THere is one AFAIK for the BMW Nav 2, and as it uses the same power connector I would expect it to work on the 26xx as well. IIRC it's about $100 though
Be cheaper to go to Maplins and buy a 12v battery pack ( as used in fire exit signs etc) and buy the connectors for the GPS and so that you can use your bike charger (optimate etc) to charge that battery. To be really clever, include in your set up the connectors needed to connect that battery to the on-board accessory socket, so that you can recharge it when your on the move.