But you can choose from many different suppliers.

Here in the US, you have to get them from the DMV, but the legibility requirements are so low. The backgrounds, narrow font, and minimal reflectivity all make them much worse than those in the UK and Europe.

(Lack of enforcement is also a problem: I lived in the UK for ~10x the time I've lived in the US, but in the US I've seen ~10x as many cars with missing numberplates.

Those aren't legal, although you can probably get away with them for a while.