Marshall PEDL-00001 one-way footswitch - $30

In great shape, except the bulb for the LED is missing.
Otherwise works fine.
This will work on any amp that has a single footswitchable feature that can be turned off or on (distortion, reverb, tremolo, effects loop, etc.
) and uses a switch with a 1/4 inch jack to accomplish it.
For example, I was using it on an Orange Rocker 15 combo amp to change channels.