you could try making them yourself...
if you can create them on the pc and then print them onto transfer paper then you're good to go
http://www.craftycomputerpaper.co.uk/category/Decals
I have used this on RC cars and candles with great success
I have used this with mixed results.
Also make sure you order the correct paper for inkjet or laser jet. Inkjet also needs to be sealed with a varnish/etc after printing, or the colors run in water transfer.
There are two colors of paper, white and clear. Be aware that light colors do not work well on the clear paper, if used on a dark surface. Darker colours are fine on clear paper. For light colors you need to use the white paper, which may not be right for your model.
If you can get vinyl, or similar, letters made that is a better option IMHO.