How to convert 5v to 1.5v?
Easy: use an adjustable 3-terminal voltage regulator and connect the "adjust" pin to ground.  The downside to this is that for every watt that goes to the motor, you'll burn up around 2.3 watts in the regulator.  This means you'll need to pay attention to heat sinking, and you'll get really low system efficiency (but it'll work).
Medium-hard: Find a step-down converter, either a design to build or a module on ebay.  If you find one that's current limited and nice and robust, and choose a current capability that's well above your motor's running current (because motors always consume more when starting than when running), it may work OK.  You may have to do some experimenting, because by default step-down regulators don't play nicely with motors.  Efficiency will be much better.
Hard: Use a half-bridge driver with PWM, with a duty cycle of around 30%.  One downside is that if you're a newbie, you have no clue what I just said.  An upside is that you have all the bits you need to make a speed control; you just need to be able to adjust the duty cycle from 0% to 30%