I guess Ebay is as good as place as any.
Maybe something this one?
https://www.ebay.co.uk/itm/FARNELL-N....c100005.m1851
Not sure what the Max current output is. It has to be HIGHER than what you need, ie ~15 Amps.
Research it or ask the seller what its rating is.
Other than that you could get a Switch Mode Power Supply cheap of Ebay. They are cheap, small, efficient but alot more complicated. Some have to have a load connected before they will turn on......
A 24V battery charger will work if it can supply enough current.
A local second hand shop or junk yard may have something?
Radiospares or Farrnel ( They won't be cheap....lol)
If you go this route, you will only have FULL SPEED unless to also use a PWM DC Motor Speed Control. ( cheap on Ebay ) and you wil ahve to turn the spindle on and adjust the speed manually...not a big deal....
There's a few options.