Cindy
You are correct with the term double throw switch. Just use a standard 3-way switch. Connect the hot to the common terminal, the disposal to one of the traveler terminals and the instant hot to the other traveler terminal. Another option would be to use an air switch designed just for this application. ISE and Franke make them. They plug into a constant hot receptacle and have two receptacles on the air switch box. One for the disposal and one for the instant hot. When you press the button it turns the disposal on and the instant hot off.
Curt
