I am trying to install a new garbage disposal. My sink and dishwasher are in an island with an outlet on the outside of the island. I would like to know how to install a switch for the disposal under the sink from the outlet.

 

You will need to run a new circuit from your panel to the island.

Ths disposal cannot be wired to the small appliance receptacle circuit.

 

As pcboss said, you are not allowed to use the circuit for receptacle on the side of the island. You may be able to use the circuit powering the dishwasher, depending on the requirements of the dishwasher and the disposal, and on the capabilities of the circuit. Provide the details and we can help sort this out.