It is still quite easy to find working batteries for N900. They may still be found even from your nearest mobile accessory shop. I found some (pretty crappy quality, but working at least) from Motonet (Finnish people know this) in my hometown. Just be sure you don't buy anything which is advertised to be "original Nokia n900 battery". All original stuff is dead already, but there's some good quality accessory/third party batteries still available.