This page is just a collection of random things, but the collection of data that I've done may help you. For more information, I will refer you to comp.sys.wearables , Wearables Central , and the various Archives .
Parts Finding the right parts is always difficult.
Here's a short list of some of them. Here's a cheap, simple schematic that you can use as a starting point for creating a self-powered serial input device. Note that this is transmit only, there's no receive capability shown here. The basic stamp could easily in an afternoon do what the above PIC schematic does. Of course, it might take some doing to make it truly serial-powered, but it is definitely doable.