Look at that foam go!
Promising review: "It seems counterintuitive to put a little packet down your garbage disposal to clean it, but this does actually seem to work, at least for getting out weird smells that baking soda and vinegar don't really touch. I scrub the sink down first, especially around the garbage disposal area, rinse it down, and then just follow the directions on the package. I have used it with single, large-size sinks and with double sinks — only difference is that the double sink will have blue cleaning foam rise up the drain, which you rinse away when the disposal sounds clear again. I have tried the foaming cleaner in a can, but unfortunately it stopped coming out of the can about 10% of the way through. Went back to this tried-and-true." —KS
Get it from Amazon for $3.98