I have a Fossil Abacus WristPDA AU5005, its basically a wristwatch that runs PalmOS 4.1 Monocrome.
I was wondering, if the battery starts becoming weak, does anyone have any idea where I can get a replacement? Or possibly any other replacement parts for it? Fossil has stopped making these.
Here's a picture of the internals of the Abacus at the right, from someone who hacked a bigger battery (on the left) on the back:
http://hamdomain.com/wrist-pda/batt-mod-1-big.jpg
The original battery is round, and also soldered to the PCB, so it's not user-replaceable. If you find the specs of the original battery (voltage, current, type, eg 3.7 volts, 300 milliampers per hour, LiIon) and size/connector types, you may find something identical at major electronic distributors, such as mouser.com, farnell.com and others. But you're still going to need it replaced by a professional.
In my country there was a company specialized in servicing rechargeable batteries for any electronic device, and if there wasn't one already made they could build one for you to the specs you provided (or sent in the original so they can use the original cables/connectors). A couple of years ago I've seen this done by the local branch of GP Batteries - they have all sorts of cells which they assemble together to make the kind of battery pack the customer needs. Not cheap, though. Check in your area or online if you can find a similar service in your country.