These kinds of adapters are available through Amazon too, along with packs of NiMH rechargeable batteries and a charger. If you're serious about being prepared for zombie attacks, etc., you should have all of the above.
And for when the zombies eat through the wiring and cut your power trying to get to your brains, you can also plug the chargers into the 12V car starter battery packs too, and charge the little NiMH batteries that way too.