Type up what you want (Numbers, names etc.) for your fleet. Print it off.
Cut them out. Best to use a sharp craft knife & a steel ruler, though it is
possible to get the numbers on to mini discs with careful use of a hole punch,
the office thing for putting holes in the side of sheets of paper.
Glue them on. I find a thin spread of a pva glue works well, then varnish over when dry.
The advantage of this system, apart from being cheap, is that you can use
different fonts & icons.