All you need is a DC motor drive. The drive takes in 120 vac 240 vac and output a DC voltage. The drives can also give a variable output voltage resulting in variable speed.
I have several used ones made by Seco or there are other manufactures.
Something like this should be what you are looking for.
Many of them have analog input to allow programing the spindle speed form the CNC controller.
If nothing else some of them have a pot on the front that lets you control the speed manually. If all you want is on and off, then just set the pot to max and leave it. The one that I have can give on and off plus speed control from the CNC controller.
Let me know if this answers the question.