My vote is for 3 x AAA batteries. When travelling most trips don't last more than 2 months so you don't need a charger unless listening to music. Of course, MP3 players these days are so cheap and small, why would I bother with the bulky reader???? In the worst case you can always purchase Alkaline AAA just about anywhere in the world.
Ideally, you will use 3 AAA NiMH (rechargable) batteries. You can still plugin the reader to recharge as now. You can even use the newer low self-discharge types such as Eneloop which can sit unused for months and still retain most of the charge. The key difference here is that if/when they become old and you need to replace them it is trivial. If/when you don't have any charging options available you can always grab regular cheap AAAs as a temporary replacement.
As it is now there are few options. You need to open it, which is easy but not every day type thing - takes half an hour as it must be done with care. Then you need to locate a new replacement battery. That can be difficult as it is not a standard format, hence few suppliers will offer them. Concordantly they will demand a higher premium, markup. Here is an example of a very similar battery for my PDA:
http://www.tangshop.com/palm-zire-71-battery.html
Price: $24.95
By the time I pay for shipping and other fees, it will be minimum $30. I could probably purchase 3 Eneloop NiMH AAAs for at most $10 at a local store. They will last just as long if not longer. They'll offer a similar performance. etc.