Lithium coin cell, 20mm diameter x 3.2mm thick. Nominal 3V, 220mAh capacity. Fresh, first-quality production from major manufacturers, not cheap knockoffs of dubious quality.
These are used in the PicoKeyer, but are also extremely common in all kinds of things. Check your car and Harley remotes, garage door openers, glucose meters, pocket calculators, LCD clocks, LED flashlights, PC motherboards and whatever else you have... why pay $2 or more each for these at the local store?
My normal stock will be either Panasonic, Sanyo or occasionally Eveready.