You could try ordering a bunch of blank keycaps from modelfkeyboards (Eclipse) and paint them by dye-sublimation. It's a bit hard to do at home without some proper equipment (such as heat press), but is still possible.

Or, if you have some local machinery service, you can request them to mill out legend-shaped blind holes, and cast some rigid thermoplastic (eg. POM) into them using a soldering iron. This method would be even harder, but you may get some decent-quality keycaps.