Speed Measurement

I need to measure the speed (rpm) of the wheels of my robot and want to use an encoder with the motors. I already have the robot up and running, so I just want to replace one of the motor with the motor+encoder kit and leave the rest of the 5 wheels as it is, with just the motor (no encoder).
Kindly help me in selecting such a motor+encoder kit to be compatible with the rest of the motors (on the 5 wheels). The motors currently on the robot are the DC motors (6V 60 rpm), with metal gearbox.
Thanks