We have both the frames and plates. Our last plates were thin, flexible plastic as were the frames. The older ones they used before my time were the hard plastic kind that can crack easy. We got them from the same place we got our t-shirts etc. It was a local business called BS Advertising. US School Supply may have them, but I'd check local first.
Do you mean real license plates or frames?
If you want a license plate in Nevada (from the DMV) and you want something on it, you have to have a certain number of signatures on a "petition" which is really an intent to buy (but no commitments) then the DMV will issue it.
I don't know if other states have a different approach...
You can do a goodle search & just put in "Personalized license plates" You will get a bunch of hits. The first one that came up was www.autoplates.com/
You will just have to check them all out to find the best price.