$18.99 Robot Marketplace

http://www.robotmarketplace.com/products/IL-SAN50.html

There are cheaper suppliers if you want to buy in bulk. Its a fairly common antweight (combat robot class) drive motor. Different gear reductions are available.

For the electronics, just dismantle a standard size servo. Remove the motor and wire in the new gearhead motor. Remove the potentiometer and wire in a new 5k linear potentiometer that is attached to the nut of the retract mechanism. Seal the servo board with heatshrink and some hot glue. Instant proportional retract mechanism that can plug right into your receiver. You can program the endpoints with your transmitter.