pali is right, you need a charger of some sort. It could be anything that uses the same kind of battery. Other phones, not just N900 (old "dumb" phones are the best), or other devices. I once had a Bluetooth GPS receiver using the same battery that could be charged from a USB port.

Your best bet though is to invest in a universal one, like e.g. this one (in your country variant, obviously). It costs peanuts and can be used for anything: cameras, other phones... Ever since I got mine I am not taking anything else on holidays.