At least a few years ago, there was a guy in the Netherlands selling adapters--basically he was carving the shells of old batteries, soldering in a resistor, and leaving space for the new battery inside. I bought a couple for my Nikon. If I remember when I get home, I'll see if I can find his info.

This is why, when I bought my own Luna Pro, I went with the Luna Pro F, even though I didn't really need the flash meter. Takes a regular 9v battery.