These motors are close to the RB35 (popular in Europe) and even come in more rpm choices:
http://servocity.com/html/3-12v_gear_motors.html
Edit: these might be the same that Espeefan suggested?
They do have a 12-24V section as well, but the sleection is much smaller thanj for the ->12V motors:
http://servocity.com/html/12-24v_gear_motors.html
Similar style motors are offered on ebay, mainly being sent from China/Hongkong, search for
gear motor.