search for hilltopper electric bike kit.
prices vary from what you want $149+, just for the motor, but basically I got a motor and built up a wheel myself and bought some of the components. I use the electric wheel to assist me (while pedaling) to go up a hill or start from stop on an incline. There is a momentary on/off button to activate the motor, or you can rig it to just turn on/off with a switch.