Take the old ones out or at least get to where you can see the battery's rating. I'm not exactly sure how many batteries a Jr. takes but if it's anything like the plus series, it takes two 12 volt, 7 amp/hr (ah) batteries. Measure the width of the battery posts and start shopping on the internet. I saw batterymart dot com sells them. YOu can also go to Radio Shack or an electronics store like Fryes. They're about $18 each. I'd opt for "heavy duty" if you can get it. When I had my 4 Plus, I replaced my batteries each year anyway. It was a small price to pay for optimum performance of my machine.
As volusiano said, you get a couple hours use per charge anyway so I wouldn't get any spares to keep around the house. Just pick new batteries as you need them. Each battery will have a manufacture date on it so I like to go pick out my own batteries and check the date on them.
