I've used stickers for a programming language popular at IBM (APL) that uses special symbols. The stickers were sold by IBM. Very high quality stuff as it's still stuck to the keyboard of my first notebook computer ( a thinkpad ca. 1993.) Granted, I haven't used it much in the past fifteen years but I used it quite a bit when it was new. Still legible and the whole thing looks very good.
Keycap stickers of various types were popular for a while. More than likely, stickers that you find nowadays probably won't be of the same quality. Certain things (e.g. writing paper, keyboards, reasonably priced loudspeakers) have deteriorated so much to the point of being just bearably usable.