Looking at this thread again, I realized that I forgot to mention another source of aftermarket decals, and that's the model railroad part of the hobby.  You might not find something made for your specific purpose, but you will find lots of decals that can be used for other things.  There are lots of sets of letters in various fonts, colors, sizes, etc.  And sheets of stripes of all colors and styles.

Here's an example.  This is a 1/1200 scale model of the USS Yorktown in her pre-war livery:

[ATTACH=full]86177[/ATTACH]

I needed to add the yellow stripes and recognition markings to this wargaming model.  Model railroad decals to the rescue!  I think these were from Microscale, but there are a lot of different companies who make them.

Model railroad suppliers are good sources of other supplies that scale modelers look for, especially diorama supplies.

And I even found a solution to a problem in the automotive supply world:  I have an anime figure in progress, 1/8 scale, and I want to finish the base like linoleum in a checkerboard pattern.  A websearch turned up exactly what I needed, a checkerboard decal from a company called StickerBoard.

I keep my eyes open, and note things that might be useful to me later.  You never know what you'll find, and how it may help when you need it.