A DPDT switch can reverse the polarity of a direct current motor if the switch is wired into the "H Bridge" configuration. In this situation, the upper and lower poles are connected in a criss-cross pattern, which flips the positive pole to the negative pole and vice versa.
A DPDT switch is a "double pole, double throw" switch. While a switch in a circuit normally connects or disconnects the circuit, a DPDT switch can be configured such that it reverses the polarity of the entire circuit. These switches may be sold as "flip switches" and can be purchased from any hardware store that sells electrical equipment, such as Radio Shack.