Laptops for Less, a reputable dealer located in the L.A. area, offers replacement batteries for PDAs. These include all those great PDAs that don't have user-swappable batteries, such as the iPAQ 3700/3800/3900, Visor Prism, Palm III and m series, Toshiba and Sony Clie. All batteries come with good instructions on how to replace the battery yourself. Prices range from US $45 to US $65.
I tried their iPAQ 3800/3900 battery and it works just fine. In most cases these are factory spec replacements, which means the same capacity, though some increase capacity slightly.
Be aware that you should back up your PDA before replacing the battery. Most PDAs don't have backup batteries, which means once the main battery is disconnected, your data will be wiped out.
This is not a paid announcement . I know many of our posters on this board have been concerned about having to spend big bucks to have the battery replaced by the manufacturer when it gets old and tired. This is a reasonably priced alternative to sending your PDA back to the factory for battery replacement.