One thing to note is that if you can't find gears just exactly the right size, you could raise that center gear a bit to adjust the spacing.
That could be a real finger-removal operation, too. I remember Sheldon Brown commenting how one of the hazards of fixed-gear bikes was that they were more dangerous to work on as the chain, sprockets, wheel and crank all spun together with a lot of momentum, and one finger in the wrong place and it was gone.