Which wheels are suitable for Power Functions M-Motor
The M-motor is directly compatible with almost all the wheels that LEGO has produced, and, with a little building, compatible with every wheel imaginable, even brick-built wheels.

If your problem is that the wheel is smaller than the diameter of the M motor, perhaps you want to attach the wheels to an axle, and then drive the axle with the motor.  There are many different gearing solutions variations that would accomplish this.

If you need to purchase one or more wheels, Bricklink is a great site to buy individual bricks.