How to mimic the blurring effect of a high speed oscillating tool.
You can buy normal DC motors (termed brushed, they have only two power pins and expect DC). If you attach an asymmetrical weight to the shaft, and have it soundly attached, your motor+weight will vibrate much in the same way you want to achieve when you turn it on.

Then the question is whether you really need the high-speed oscillation like the 21000 RPM you mentioned. It sounds a bit of an overkill to me. My advice would be to first try with the regular DC motors. They are usually limited to 6000 RPM, which is 100 Hz oscillation-wise. The human eye definitely wouldn't discern this movement - it would look blurry. If you really need the 21000 RPM, you can go with brushless motors, as their RPM limit is much higher. You can search for motors intended for remote-control cars/helicopters, and you should buy a matching controller as well, since they are much harder to drive (but once you get the controller - the ESC - it's again piece of cake).

A bit more on your question:


  Why do motors get the RPMs they do


Brushed motors contain an interface between the stator and the rotor; you need to pass current to the rotating part, as well as to periodically switch the direction of the said current. This interface is called the commutator, and a set of graphite brushes act as a contacting material. They have to endure the friction. Running a brushed DC motor above its rated RPM leads to excessive sparking over this interface and the brushes wear out quickly. So the speed limit is not absolute, the motor can exceed it, but it's not recommended for its longetivity.
Brushless motors don't have this interface, as the phase reversal and timing is done by the external circuitry - the ESC. This makes them both more durable, and also capable of much higher speeds. I don't know what is their limiting factor, but it could be purely mechanical.