I've had luck at my local Target and drug stores for oddball Lithium
batteries.
Just a quick note that can sometimes help when trying to find
batteries-- On lithium cells such as these, the numbers are *very*
descriptive. The first two numbers are the diameter of the battery in
millimeters, and the second two numbers are the thickness in tenths of
millimeters. Therefore, if you have good springs in your battery
holder, a 2320 will fit in a 2325 holder pretty well. If you put a
2025 in there (the most common size, from what I find) you get 3 mm of
play, but the springs hold the battery in pretty well. Long as you
don't lose electrical contact, it should be OK.
I discovered this with my ancient Casio watch. it takes a 2730 (gack)
battery that is nonexistant any more. I put the biggest battery I can
find (I think a 2530 or something like that) in the holder, and it
works just fine...
All voltages/currents are pretty much the same across the lithium
battery line; bigger ones seem to hold their juice longer, but size is
the only determining factor as to what a particular item can use.