I am guessing this is a really old beast or HP wouldn't have discontinued the battery for it. If you can't find the exact one on eBay, you are just going to have to take a walk on the wild side and try some 3rd party off brand battery replacement. Buy it from Amazon so they will back you up if the thing doesn't work.
Moreover, buck up and get a laptop that is closer to this decade. You can get a fairly hot basic one for around $169 these days. I am guessing it will be light speed faster than the one you are trying to salvage. If the HP was some hot gaming computer then a new one will cost you $$$. If it is just for gaming, leave the thing plugged in and forget the battery. Take the tax write off on the old laptop and go get yourself a...