Hello. Welcome to Just Answer.
Most likely, there is a loose connection at the switch. It's not uncommon for lights, or anything else connected to wall switches, to behave this way when a connection in the switch box is loose.
The first thing I would do is try to find the breaker that feeds that circuit, and turn it off. Then, pull the switch out of the wall, and examine the connection points of all wires. If the wires are connected to the switch via the small push in holes on the back, I strongly recommend redoing the connections using the screws on the sides.
I don't think the switch is bad, I did further investigating with a meter, I think it's the outlet box under the sink. The disposer works when I plug it into an extension cord from another outlet. I will have to get an electrician for this one. Thanks.