I'm sure they make decal sheets full of letters and numbers. Whether or not they sell it where you live is a different story.

I'll see if I can find one online...
They'd look like this:
Code:
AAABBBCCCDDDEEEFFFGGGHHHIII
AAABBBCCCDDDEEEFFFGGGHHHIII
AAABBBCCCDDDEEEFFFGGGHHHIII
JJJKKKLLLMMMNNNOOOPPPQQQRRR
JJJKKKLLLMMMNNNOOOPPPQQQRRR
JJJKKKLLLMMMNNNOOOPPPQQQRRR
etc...
And you just cut out the ones you want and slide them on one at a time.
I actually saw a sheet like that, but it had all the characters you need to make Japanese license plates, including the
kanji for the different prefectures. That was cool, too, since the Japanese plate that come with the kits always have numbers like 20-00 and 18-00, usually to match the size of the engine.