I dunno which kinda of button you mean, but you can get some buttons where the cap comes in two parts that click together for you to add a label or symbol in between.
You just use a ink or laser printer to print text on some paper, card or OHP film then cut it to size and put it inside the button cap.
I was hoping for something that I could put in front of a 10c tactile switch rather than a big heavy one, but you're onto something there, I think.
While I was before unable to find an appropriate cap to put printed text behind, if I have to get something custom-made a similar method would work---two pieces of plastic with the text sandwiched between, and either glued down or with the rear one a few millimeters larger to keep it from popping out of the hole. Such a method might even lend itself to handworking, though getting Ponoko or someone to laser-cut the material isn't terribly pricey.
You can use a label maker with clear label material to create your switch legends.
My fear there would be that durability would suffer, but this is probably not a bad option.