They aren't plastic, more of a thin-pressed translucent felt. Well, I suppose the material probably is a plastic filament of some sort, but it's more like a thin liner material for clothing or drapes.
When I reworked my D-50 several years back I got replacements from Rob Senso at Vintage Planet but he shut that down of course. They'd be a time-consuming b!tch to make by hand as they're wide-slit and different widths, but it could be done if you were determined enough.
If you'd like I could send you the old Coke-stained set from mine as a pattern (I never throw anything away!) or you could try washing and bleaching them for reuse.
Have you tried Roland parts department for grins? They never have anything though...
Does anyone know where or how to get a hold of the little plastic key guards that went in D-50s and other Rolands of that era? The ones that are glued under the keys. I had my D-50 cleaned and serviced by a local tech a couple of years ago, unbeknownst to me he threw away the plastic guards when he cleaned the contacts, I only discovered this after I recently took it apart to replace a broken key. I know it's not that big of a deal and not having them doesn't effect the keyboard at all, but I am a completist and it is driving me nuts. Well, I think I just won the most random question of the week award on this forum! 
