I have an old laptop with a kaput battery.

I have looked at this question with regards to spares, but most of the sites that are linked too from there have batteries which probably cost more than the laptop is worth. I like keeping the laptop around as a spare, but find it fustrating that it has to be plugged in permanantly.

It seems to be that a half good battery would be acceptable for me, for a knock down price. However nothing of the sort seems to exsist.

Is there any way to get cheep batteries in such a case?

Laptop is a Compaq Presario 900 if that information helps