I picked up this Digipower DPS-9000 external battery for my digital camera a couple months ago, but just realized that it could be used for the Axim as well!
What it is: a tripod-mountable, semi-pro Li-ion rechargeable battery for Canon, Casio, HP, Minolta, Ricoh, Toshiba, Agfa, Epson, Kodak, Nikon, Olympus, Vivitar, Fuji, and Sony [though not "all" sony's] cameras (it includes all of the necessary adapters). Also has a "fuel gauge" on the side that shows the amount of charge left (see picture).
How to use with Axim: Just so happens that the Fuji adapter (which puts out 5VDC) has the same plug and polarity as the Axim. The latest intelligence tells us that the rated capacity of the battery is 1800mAh.
Other good stuff: The battery comes with a wall and car charger, it charges very quickly (couple hours), it is lightweight, charges the Axim fast (same as Dell wall charger), and I can get at least a couple recharges on one charge of the battery! And of course, Li-ion has no memory effect so it is easy to use. Wow!
Where to buy: Watch out! I've seen this battery in photo magazines for $100, and CompUSA sells it for $79.95. Although it might be worth that, Best Buy sells it for $39.95!
[UPDATE: now only $21.95 at Best Buy if you can find it.] Nothing more you need to buy. What a steal! And of course you can use it for your digicam as well.
Hope you find this info useful!
Shout outs to pyro, Agentrapid and Wayfinder for their most useful input and research.