Swapping batteries or getting a custom made one seems like a bad idea. Especially as those battlepack.com guys appear to do NiCAD and LiIon, and car electronics typically expect lead-acid batteries.
But digikey comes to the rescue with some open-frame DC-DC converters:
A very flexible TI one
A cheaper, less flexible one
The TI one looks like it has better transient responses and a wider input voltage range. Which you'll need on if you're running this with an alternator as well, since the electrical system's voltages can vary quite depending on loads. At any rate, for either of those you'll need to come up with a modest enclosure to protect them and read through the datasheet to make sure you're giving them everything they need to be happy. Specifically, they need a particular resistor to set the output voltage and capacitors on the inputs and outputs.