No joy at Rona or camera shops. Also checked out the PX27 specs, but they are too large (12.7 mm diam.)
I think I may have identified it though! There is a 6 volt alkaline battery called a "GP-11A" which is used in remote car alarm transmitters and garage door openers. This battery seems to be mainly used in the UK, and much more rare in North America. GP-11A is the most common name, but they are also called 11A, G11A, or MN11.
The diameter is right on at 10.0 mm. They are a touch long at 16.0 mm, but still in the ballpark. I'm a bit worried about the voltage at 6 volts, as my old one was putting out 3.6 - but of course it was half dead... Does anyone have one of these sights with a fresh battery that they could check with a volt meter?
Here's one site that sells them. Note that this is an FYI, and not an endorsement. I haven't tried them yet, and as I mentioned the voltage could be a bit high. I will order a couple and try one out though, either from this seller or somewhere else...
http://www.batteriesinaflash.com/ind...roducts_id=219