Most battery packs that I have dissected are made up of a configuration of 18650 lithium cells. This includes all battery geek packs, wally world DVD batteries, and the sony batteries used on our SD 7xx recorders.
http://www.batteryjunction.com/tenergy-18650-tab-2600.htmlI had a good website saved, that I lost on my last computer crash, that explained how to identify the country of origin and the capacity of these cells by the color and the numbering.
If I recall, the best cells were from Japan and Taiwan, the worst from China.
These cells are relatively easy to find and work with, although they have more than double in price over the last few years. In most cases the cells would need to be soldered together.
Chances are if your battery pack dies, it is from the cells going bad and not the charging or voltage regulating circuits. So by installing new cells, you should be able to bring a bad battery pack back to life.