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!