I would suggest looking online.
You can get the measurements from the battery box, and order one that will fit online from Batteries.com or other sites.
It's a 12V and I have the 3.5ah SLA batteries (i have 2) that i added the quick adapters to (easy solder) and made a car charger so that i can charge the batteries on the run. Each battery runs the call for days on High power.
I get them online for $15 Each year I replace an older one, with a new one. But, you can get a couple of seasons out of them as long as you make sure to charge them on a regular basis.
For those that don't know, you really can't use the high power mode with D batteries. It uses 8 at once and will suck them dry in a day of calling on high power. The SLA will last all weekend and not have an issue keeping up with the needs of the call.