I ordered some plates for my car in the original 66 license plate # (I lost the originals years ago when I took the car off the street) from this guy (
http://www.cr8apl8.com/) and the Ca DMV people asked me what I was trying to pull. The plates have to be original California plates not reproductions in order to be assigned to a particular car of that model year. I ended up getting Historical plates which limits me to special events only.
DMV told me that if I bought real plates at a swap meet and they were clear on their books I could register them to my vehicle. I was also told that I needed both plates for them to inspect. I guess I am a dummy for not reading the fine print on the cr8ap8 web site because when I emailed the guy asking why he was selling stuff that couldn't be used, he called be a ##$###$ in several new and creative ways. I told him that it was a not very smart to treat a customer that way and he told me he had tons of orders and didn't need my or any of my friends business. Bottom line, you can use cheap vacuum formed replicas at car shows but I wouldn't take a chance with them on the street. I understand that if the officer wants to, he can impound your car if the plates are not real.