I was looking at some generic ones on eBay for around $50. But like you said, it seems a little sketch. I'm afraid of getting some generic battery that will fry my iBook. Then again, the iBook is so old that I really don't want to invest over a $100 in a battery for the laptop.
If the iBook still does what you need it probably makes sense to spend the money for a battery. But there definitely comes a point with these old systems where it's better to put the money toward a new one instead.