that there are NO 1/18th 'alphabet' decals. What you need to search for is a particular type of typeface and size in either 'scale' dimensions or type face point size
If you aren't looking for script type text, only individual letters, you might have some luck looking at model aircraft decal sheets.....especially WWII, as those aircraft used some rather large typeface.
You need to decide what size ( in real life ) you need your lettering to be....1/8, 1/4, 1/2, or something else, then start looking at sheets that meet that requirement.