I can get SVR Audio batteries like the SVR-33 or SVR-28. The 33 is cheaper and less reserve, but is most common.
I agree a second battery should be used first then add the cap. Usually though the caps and batteries are around the same price, I know I wont be able to get any battery for this price,
Lets see how many are intereseted in the SVR Battery:
www.svrbatteries.com