Look around Ebag for variable frequency drives. These are getting cheaper and cheaper. Another route is to troll the thrift stores for a used "older" powered treadmill. You want one, if possible with a basic speed control with a pot--like a volume control.
I've got one on my little Atlas 12" lathe, and it works great. These are a DC variable speed motor.
Yet another way is to simply drive it with a variable speed drill such as the earlier "junker" photo I posted.