Radio Shack is probably the easiest place to buy one ... just ask for a 5K (5,000 ohm) potentiometer.
If don't know what they look like, do a google search to find out. A pot is just a variable resistor... 0-5k ohms in this case.
make sure y ou get a LINEAR taper...not an audio taper... Linear means the speed goes up "in sync" as you turn the knob up.
Audio taper is based on Db's, which is logmarithic scale -- a small turn at the middle to end of the rotation means a LARGE change.
New pots cost around 3-4 bucks at Radio Shack.... any surplus electronics house (look in t he yellow pages) will have used one for 50 cents or so.
And be sure to wire it correctly.. usually the left most terminal WITH THE SHAFT UP AND FACING YOU, and the pot turned all the way counterclockwise is wired for zero speed, the middle terminal is the wiper and the rightmost termimal is wired for max speed. Use a simple ohm meter to check it, although if you are not sure what a pot is... you probably don't have one...
I had the same issue. You need a 2 watt 10 K pot linear taper with a knob and legend plate.You can get them from McMasters or Mouser for a few bucks. Radio Shack doesn't seem to carry many components just a small selection. Too little profit from small sales I believe.
?Ref=4408&Row=577544&Site=US is probably a good match. Allied Electronics or Mouser would also likely carry them. You will want a linear pot (not logarithmic), the other common choices will be related to style of mounting and shaft type. These can often be found surplus for $0.50 or so.