Congrats on the 35SP...its a great camera...I have one too and also the older 35-S...the lens is fantastic...super sharp and buttery smooth...
About the battery I found this on Andrew Yue's site...
As far as I know, the 1.35 volt mercury oxide battery cell that is used in the 35-SP is no longer being made. That's a shame, because mercury oxide cells, in addition to being long lived, have a very even and reliable discharge voltage that's a perfect match for these old CdS light cells. So, it may pay do a search for either the PX625 or PX13 button cells while there are few left somewhere. The more practical alternative is to try a zinc-air replacement for the mercury oxide cell. Wein Inc. markets a Zinc-air replacement cell that is a drop-in replacement, but has a limited life span once its seal is opened. I'm currently using a Z625PX zinc air from photobatteries dot com and it works well. In the humid climate where I live, they last about four to six months. With a little ingenuity a size 675 zinc-air hearing aid cell can be made to work for about a $1 a cell. The more permanent solution is to have light meter calibrated to accept a silver oxide battery cell. This usually can be done while having the camera cleaned, lubed and adjusted, CLA'd, at a reputable repair facility. Silver oxide cells are widely available, and provide a constant voltage over their life. However this voltage is about 1.55 volts, so you can't just drop one in and expect to have accurate exposures. In case you are considering the use of alkaline cells, be aware that their oultput voltage will decline significantly during use, resulting in an even wider range of inaccurate meter readings.