wings, I've also seen the inserts on ebay for just a few bucks but watch the shipping charges.
I also just checked tower ya got like 17 to choose from.[&:]

Flyingmonkey, any 12 volt source will work. I used a motorcycle battery in a homebuilt box for the last few years and it worked great.
In the pic the battery is under the panel built in and stuffed with foam.
When the battery was new I only had to charge it about 3 times a year.