Try a Camera shop for a 12V PAG Light Battery Pack.
They're very powerful and last for a long time with a 12V PAG Light attached to a Camera or grip. They should last for hours on a GPS receiver as the current will be considerably less.
You'd just need to make up a lead with a male XLR connector to a cigarette lighter socket (which I think you can get from Halfords).
Hope this helps
FWyg
