I have some good news for those of you who, like me, still have a clock that works. I bought a new LR50 battery on Amazon.com (about $5), works great. The advantage, as was mentioned above, of this battery is that it has a contant voltage right up until it fails, as opposed to an alkaline battery.
If you have the clock with the timing adjustment screw like mine does on the front of the clock you can adjust it to keep good time even if you use an alkaline battery, but as the voltage gradually drops you will have to adjust the timing from time to time to maintain good time.