I see four different things on Ebay:
1. One guy is aluminum plates with vinyl letters, usually spelling out something in English for $23.
2. One guy selling stamped aluminum plates which all have the same prefecture, vehicle class code, and prefix, and four random numbers for $23.
3. On guy claiming "authentic" with stamped aluminum plates, with a couple different prefecture and vehicle class codes (limited and you can't choose), and they all spell out a car related phrase in English, some in f-6 letters, for $23. (Not very authentic.)
4. One guy selling genuine used plates, you choose from what he has to offer, and they go for $35 for singles or $80-150 for a pair.
At the risk of giving away a good secret, I found two custom plate makers:
Has only the pattern for Nagano Prefecture.
You choose the vehicle class code, but they only allow two digits, so three digit codes can not be done.
The prefix available is SE.
Up to five digits for the identification number, so you can do two numbers, a dash, and two numbers, or two numbers, a space, and two numbers, for an authentic looking plate.
The number font is narrow like the real plates.
Embossed aluminum and not drilled, so can be drilled for US or Japanese bolt spacing.
$35 per plate.
http://www.licenseplates.tv/cat/japan_l ... lates.html
Standardized plates available with Fukuoka, Hiroshima, Kanagawa, Kobe, Nagano, Saitama, Sapporo, and Tokyo. Only two character kanji for the prefecture name, though some prefectures actually use three kanji characters on their plates. Several different plate and lettering colors (motorcycle too.)
Choose up to three digits for the vehicle class code.
No choice of prefix, determined by the standardized set matched with each prefecture.
Choose up to six digits for the identification number. So you can get the two numbers, dash, and two numbers, or two numbers, space, and two numbers.
The number font doesn;t look right, too wide in the example pictures. But that might be just in the example photos.
Embossed aluminum but pre drilled for US bolt spacing.
And a fully custom reproduction option from LicensePlates.TV
You supply a picture and dimensions. A photo of a real plate, or draw a plate with the prefecture name, vehicle class code, prefix, and identification number you want.
Reasonably sure the font matches your drawing or photo and either drilled per your instructions or tell them not to drill it.
Starts at $100 per plate.