thanks for pointing me to 3dhubs - it is just the service I was looking for. Today I received my Inon knobs made of glow-i-the dark filament; they fit perfect.
Just a hint: When ordering these knobs it may be a good idea to ask for 100% fill (=solid). Seems that a standard fill is only 20%, so there might be the possibility that the knobs will crack or lose shape or whatever deep down. I was charged less than 20 cent extra per knobs.