You can make your own.
A friend of mine made some battery boxes using 50 cal ammo cans and 12-volt burglar alarm batteries. They are very nice compact units that will provide 14 amp hours of power. I have built a couple of them and they work well. I have used them to power 12-volt lights, but they have many other potential uses.
What they consist off is a 50 cal ammo can in which you drill a hole for the female half of the cigarette charger. The hole should be about 1 ½ inches from the top of the can. You then take two 12 volt 7 amp hour batteries and wrap them in Saran wrap. Place the Saran wrapped batteries in the can next to each other. Fill the can with spray foam insulation. It comes in a spray can and you can get it at Home Depot. You don’t want it to cover the batteries, just hold them in place. The reason for the Saran Wrap is so that in the future you can exchange the batteries without removing the foam.
Now complete a simple parallel circuit running from the two batteries to the cigarette charger. Place a 15-amp fuse in the hot side (the red wire) just before the charger. There are a couple of different ways in which you can charge the box. One way we have used is to take an old 12-volt wall charger and attach the male half of a plug that will fit the cigarette charger. The second way is to use a solar charger. To preserve the maximum life of the batteries the rule of thumb is to never charge more than 10% of the charge of the battery. In other words, these two batteries total 14 amp hours, so you should never charge at a rate of more than 1.4 amp hours. This is about a 20-watt solar panel. If you are using a bigger panel, you will need a controller.
We are currently working to improve these units, if you have any suggestions they are welcome.