Get yourself a speed controller for routers and try it. It is a type of a dimmer circuit made for inductive loads. They are not very expensive and should do the tick.
But how well it will work depends on what type of a motor is in that sander. Some power tools (such as table saws, drills and other rotary tools) use motors with brushes. Those respond very well to devices which control their speed by changing the voltage (which is what dimmers and rheostats do).
The other type of a motor is an AC induction motor. Those are used in bench grinders, band-saws, ceiling fans, and other tools. These are brushless motors. The speed of these is based on the AC frequency (60Hz) of the power they are using. To truly slow them down, you would need to lower the frequency of the AC power feeding them. These can be slowed down somewhat by reducing their input voltage, but this is because they simply start slowing down under the load (if they were left free spinning, their RPM would be the same at full voltage and at half voltage.
I think that there are speed controls for those induction motors, but they are hard to find and fairly pricey (as they actually have to generate a variable frequency voltage at a current high enough to power the motor.
