For the California folks....
Cal DMV won't let you register with the repro plates & are cracking down on those making them.
However, you can do YOM plates (Year of Manufacture) plate reassignment of any "real" plates for a car, trailer, motorcycle & non-commercial truck up to 1969, & commercial plates up to 1972....but you have to find & buy the right plates in a full set (2 - except trailer & MC), with the correct year sticker on it, then go through the process. I'm in the beginning of that process now for our "new" 1960 Avion trailer, & will report back in a new topic when I'm through it.
There is also the new Cal DMV Legacy Plate program, which would apply to our 914 MYs 70-76, where DMV will be making vintage looking reflective plates in variuos styles where you can also pick a custom phrase/numbers like the other custom plates. They're doing pre-orders now & will start making them as soon as they get 5000 orders, or will refund/return your check if they don't get enough interest by 2015.
Details here...
http://www.dmv.ca.gov/legacyplates/index.htm Here is another "custom plate" source that I've found out there, but have never used them myself - so exercise due caution...
http://cr8-one.com/