I need to make a circuit that will allow me to turn off, on (forward), and on (reverse)

This is done with 4 AAA batteries and a DC Motor.

What parts will I need? What would be the best place to buy the parts?

Will I need a DPDT switch? What else?

Also, is there a place that sells small plastic gears?
Worm gears, pinion, etc

 

A DPDT switch that has a center off position will do for the motor.

Gears - http://www.sciplus.com/

http://www.wmberg.com/

 

Is there anything else I would need to make it work?

How would I wire it?

 

Here's a simple schematic using batteries and a DPDT center off (ON-OFF-ON) switch:

Most DPDT Center Off switches have the common terminals in the center. However, use a multimeter to verify your particular switch before connecting the wires.