yo...you need a 12V acid-lead battery. those one have a very high capacity per volume and you can get them in various size and shapes:
but the trick is, how to charge such kind of a battery, which you can find here:
http://www.batteryuniversity.com/partone-13.htmthis is very inportant for the lifespan of the lead-acid battery! there also exist complete charger modules, which take care about that matter if you can't handle the charge characteristic manually.
then 2ndly you need a 9V voltage regulator. the easiest and confortablest way is going with the good old 7809. it's a linear voltage regulator, so the 3V he has to drop down, will turn into pure heat!! but never the less, there aren't much other possibilities to drop the 3V, unless you will go into switching step-down converters, but which are also about 10 times the difficult to handle.
here a very simple circuit for the 7809:
(*note: there is the 7808 shown, but which is handled the same way as for an other regulators of the 78xx serie).
put all together and finish is your personal LT Power Bank!!