I`ve rebuilt many battery packs, they aren`t hard to do if you have a little electrical knowledge. The new cells should be easy to find online, especially if your in the US. Try (www.batteryspace.com), they`re my favourite as they ship all the way to me in Singapore.

Stay with whatever type you are using, eg. NiMH or NiCAD (as the charger may not be suitable for different typs). But you can change them for higher capacity without any problem, so you`ll get a longer run time.