Ebay is your friend. Lots of third party products available. I've bought replacement batteries from australian-based suppliers for two different IBM laptops and they have worked well enough. I think for around half the price of the genuine IBM product. Buy a couple?
If you have a big battery or two to power your rig, a DC-DC convertor to change the battery's 12V into what your laptop requires isn't too expensive from Jaycar, eg:-
http://www.jaycar.com.au/productView...r&form=KEYWORD
On special at Jaycar for $30 now. I use this one.