How can I replace the battery. I can't find one anywhere

How can I replace the battery. I can't find one anywhere

I can't find the battery replacement for the Roomba Pro Elite which my friend just gave me because she doesn't use it. According to the on-screen manual, somewhere I saw that it is a Roomba Pro Elite 500? I can't find a battery listed anywhere for that series. According to my friend, she has had the Roomba for about 15 years. Help.