You can buy some fancy stuff, or you can use Grandma's method with baking soda and vinegar.

Find the sink stopper (not the drain sieve) and have it ready.

Pour ½ c baking soda down into the garbage disposal. I use a funnel because my disposal opening has these rubber "flaps."

Pour in ½ c vinegar. Use the cheap white stuff.

Immediately, put in the stopper. A little might bubble out; don't worry.

If the disposal is still smelly, repeat.

Not necessary to run water through afterwards because the baking soda and vinegar made water!