I use batteries designed for the Mazda Miata. They are a little smaller than the Interstate 911 battery. You can get them in AGM versions for around $100. I have had very good service with these.
that voltage sounds low. I would check the cables and see if you have any problems there. Also test the CCA on your batteries with something like this.
Digital Automotive Battery Analyzer
I have one it is a great tool to let you know if your battery is up to snuff.