I don't know if there are multiple revisions where the CMOS battery located, but this information is from my 5110H, built in September 2003.
The CMOS battery is underneath the motherboard, between the main battery, memory, and wireless card. The approximate location is shown by the green circle in the picture below: You might be able to just see it by opening the memory cover and looking sideways into the case.
To actually get to it, I think you will have to take it completely apart and remove the motherboard. I don't think there's enough room to pull it into the memory bay, but I guess you could try that first.

The battery is connected to the motherboard with wires and a connector, rather than being in a holder. It's stuck to the motherboard with thin double-sided tape, but that comes off fairly easily.
The wires run around the right and lower edges of the thin black plastic chip cover shown above, to the white connector at the lower left corner of the cover. No need to peel off the entire black cover- you can lift the edges to get the wire out.
Since my battery is still good, I didn't cut open the shrink wrap around it. So i don't know whether the wires are welded/soldered on or if there's some sort of holder inside.

They are tac welded to the battery under the shrink wrap. Battery and wires may need to replaced as a unit

Radioshack is the best bet.. beleive it or not your grocery store may even have one. It's just a watch battery.