You've heard of Ebay?

Another route is to make a servo out of an ordinary DC motor. See:
http://truetex.com/servomod.htm

You'll need more torque for ACME vs. ball screws. Bigger motor or belt down.

--
(©¿©) An apple a day keeps the doctor away.
Two apples a day gets the doctor's OK.
Five a day makes you a fruit grower like me.

Karl Townsend