Thank you all! Apparently I have posts. And just to be clear, I am talking about the two batteries outside that start the bus, not the interior ones that run the lights and stuff.
I haven't found ANY batteries less than $100 so I think I'm looking in the wrong places (online). If anyone knows who is selling the less expensive bus batteries, please let me know.
I did find a very cool link that some other people who are also super new to this might benefit from. It explains all about batteries. http://www.vonwentzel.net/Battery/00.Glossary/