im guessing the motor vechiles departments - the departments that issue you with your license and regestration

Oompa Loompas

The State legislatures generally approve any plate designs - and designs are generally done at the request of the governor. The designs themselves possibly come from a variety of sources, either within the Motor Vehicles Division of the state or sometimes outsourced.

In MO the department of revenue submitted several designs to be voted on by the public via the internet, We get new designs this year.

The department of transportation designs them and at certian times they do take a vote from the public which ones they will choose.. other then that there are many places you can get the so called "vanity plates" where your guess is as good as mine who makes them... too many to choose from....