Try either an external ICOM radio battery, such as is used with Communica or Lynx intercoms, or make your own up with a gell-cell battery from somebody like Maplins.
These high capacity gell-cells get used in everything from microlight aircraft to fire alarms, any major electronic supplier will have a large supply. Do be careful about charging them however, you need a special charger which charges at a slow rate then cuts out otherwise they tend to overheat, distort - and a spherical battery whilst very pretty is a pain to secure in an aircraft.
Quite a lot of light aircraft have 12V cigarette lighter sockets, I think that you've just been unlucky.
G