As I found out last year with my wife's CTS, you cannot go to the local auto parts houses and buy an OTC battery for your CTS. The same holds true for the STS as I found out last week. The series of the battery is 101 for the STS. A dealer item only. They go for $178 and up depending on what dealer you talk to.
The parts houses will not sell you any sort of a replacement or try to fit something in of comperable quality saying "The incorrect battery was not made for that car and cannot warranty it." They are scared to even try.
I got in the yellow pages under "Batteries" and located a few businesses that stocked car batteries. I remember a place that my Dad used to buy Semi-truck batteries at. They looked it up in their books and came up with 101 as the replacement. They installed a battery of the same dimensions with the exception of the replacement battery being about 1/2 inch taller than GM DELCO coming out of the car. Had the same CCA as GM. Also the battery warehouse said, "It did not matter what car their battery went into, they would give the full warranty for it." Not sure of the series number of the battery they put in but it fit in with little problems. A tight fit but no problems. Even the hold down mount matched up on the replacement battery. Cost less than $70 with tax. It was the same on the CTS last year.
The new Gel Batteries-Optima, etc. are a line of thought for future replacement.
What else are other doing to solve their battery suitations?