The battery can be found at Radio Shack and most electronics parts supply houses and true computer repair shops for $6-15, depending upon how much they gouge. It's a common battery type, but is soldered in place. A 20 watt soldering pencil does the job easily, and requires little technical expertise beyond basic soldering skills. Observing polarity is the only requirement, takes ~15 minutes. Toughest part is splitting the case!
The battery comes in two flavors: the original (ni-cad) and nickle-metal hydride (ni-mh). The ni-mh have less "memory" to them, so they should last 12 years, as opposed to 5-7 for ni-cad. The cost difference is $4, but I seriously doubt your system will still be in use in 2015!