Sandy, how big are those number you have in mind?
If those letters and numbers are not to small, it is an option to cut masks with a cutting plotter and use the airbrush. So also different paints can be used, not only black and white.
I have done that recently for a friend asked for it. He was able to find the right font used by RAF in the internet and told me the exact size. The painted on result looks great, his model is on scalemates:
If there are special fonts and sizes are used then it is hard to do a general decal sheet, masks are much more adaptable. You can use every font and sizes, only very small letters like stencils are a problem. And it can cut also other things like perfect circles and so on.
Due to huge interest (also by woman cut greeting cards and so on from paper) the prices are higher now for the same device (one example "Silhouette Portrait": 5 years ago below 120€, some shops offer it now for more than 300€)! So I used a special offer on a Black Friday: 139€
You can find them in used state cheaper.
Cheers,
Frank