To follow up my comments from yesterday, and to second what Mike said, after having my own Canonet QL17 GIII for a year or so, even with proper mercury batteries, and a fully functioning linear meter, I've found the meter easier than most to 'fool' in some situations. In other words, don't get too worked up about trying to find the ideal battery or adapter to make the meter work. To be quite honest, knowing how the meter in the Canonet is easily fooled, I usually go fully manual with the camera using 'sunny-16'. Believe it or not, sunny-16 is usually more reliable than the meter, especially in odd situations! Other times I do use the meter, but take a reading off of a small grey-card or the palm of my hand just to be sure.
So, that said, IMHO just go out and get an alkaline battery, then 'calibrate' the Canonet's meter against a reliable hand-held meter...It's about as good as it's gonna get!!!
The meter, though, is about the only weak spot on this camera. The QL is about the best thing ever, the lens is fast and sharp, the patch is really bright and for $10 you ought to be able to find a Canolite D to go with it which works surprisingly well. They are brilliant little cameras. Mine is always getting use.