Radio Shack has them. I recently built a power supply for a trip from Canada to Japan out of a plastic toolbox, a RS cigarette lighter socket, and a 12Ah SLA battery.
Though it was really heavy and almost got confiscated at the Seattle airport, it did keep my computer running for 10 hours.
Hint: there are TSA people at the airports who are suspicious of things that they don't understand or have never seen before.
As an interesting contrast:
US (Seattle airport): 20 minutes, had to talk to 3 TSA people to finally get the battery on the plane. This despite the fact that the battery was clearly visible, along with its rating and other markings. Questions like "what is that?" and "why would you make something like that?".
Japan (Kansai airport, Osaka): 2 minutes. One question: "is that a wet cell battery or a gel cell?".