Some hobby shops sell number decals. Tower Hobbies for example sells them for $3 for 1" decals all the way up to $7 for 3" decals. If you go to one of your local arts and crafts stores you can usually find the individual numbers. Usually you can get the numbers you need for a few cents.
http://www2.towerhobbies.com/cgi-bin...?&I=LXB264&P=7
--Scott