One- They are listed in the documentation on page G-60.
Two- Don't know, but there should be a number of places that sell them.
http://www.RadioShack.com/ sells a very similar battery. Search for part number 960-0453 on their site. It's $63.99, and I'm sure shipping isn't cheap.
Three- You probably can't, but you can put three 12.volt batteries in series.