Yeah, the stock battery model is the TX9-BS series. Get a reputable lithium battery that has at least a 3 year warranty.
And don't forget to buy a lithium specific battery charger. These lithium batteries require specialized charging cycles to maximize their life span, and only a lithium specific charger can do that. Optimate chargers are what I use.
Yeah, you could get away with using a regular battery charger for these lithium batteries, I know it worked for me. But will it really extend the life of that expensive battery? Probably not, so if you want to get the most value out of it, get that lithium charger as well.