It's a shame that mercury cells are no longer available. At 1.3v each(and a reasonably flat discharge curve) a stack of 3 would likely do a good job and last a decent amount of time. Apparently the VA here in the United States horded a bunch of mercury cells, as my grandfather had a big stash of them(for his hearing aides) still in VA envelopes with recent postmarks when he passed away-I wish that I'd had the foresight to grab them.
You might consider stacking a pair of Silver button batteries. You would still be at 3v(or probably closer to 3.1v) but the discharge curve is a lot flatter than alkaline cells so I suspect that the "real world" life would be longer. As silver cells are often $1-2 each in bulk, however, this may put you in the same ballpark price-wise as buying "the real thing."
I use zinc-air cells in old cameras designed for mercury batteries, as they have a similar voltage and discharge curve to Hg cells and thus will give correct light meter readings in camera with unregulated circuits designed for mercury batteries. Unfortunately, they are a less than ideal solution for a PRAM battery as once they are "opened"(by pulling off a tab on the battery to expose the electrolyte to air) they only last about a month.
For folks here in the US, this is an exact replacement for the battery that Apple used. In fact, I've come across some(seemingly original) batteries in systems with this same wrapper. Although it's not super cheap, it is at least available on the shelf pretty much anywhere you go in the US. I'll also add that if you use the Radio Shack "battery finder" tool it will steer you toward a different p/n battery that is the same price but lower capacity(850mAH vs. 1200)
http://www.radioshack.com/saft-ls14250-3-6v-1200mah-1-2aa-lithium-battery/2301243.html#.VIfinId4QeM
One final thing-with as many old Macs as I have lying around, I long ago gave up trying to keep the batteries fresh in them. My computers that get used every day stay plugged in all the time, and I deal with resetting the clocks as necessary on the rarely used ones. The only ones that generally get a battery replacement are the ones that either require it for proper operation, or that get intermediate use to the point where they're not always plugged in but resetting the clock is annoying.
