i haven't built one myself, but i'm guessing you could get similar car adapters anywhere.
as for the mah's, i dunno that one myself, i'm pretty sure you have it right - well kind of, the more mah's the longer it'll run a device.
i'm bad with power stuff, i just follow the how to diagrams. haven't built the thing for the jb3 yet, but i'm pretty sure everything discussed about making them is in those two threads.