Head here to Woodland Senics: http://www.woodlandscenics.com/
For just letters/numbers for the stands and on vehicles, check Accessories---> Model Graphics.
For more lettering, warning signs, shop signs, and other stuff, check Accessories---> Dry Transfer Decals.
These are the brand I use for my custom GI Joe and Transformers. They easily rub on using the blunt end of a brush and you might want a clear sealer such as Tamiyia clear to keep them from wearing.