If you just need the colored translucent plastic, you might be able to find something at an arts & crafts store that you can use right away.
If you need the exact items with the little symbols printed on them, and you're patient, your best bet is a salvage-yard, or check the Parts For Sale forum.
I've acquired some spare instrument clusters for some of my W123 projects, from the Virginia Beach Pick-N-Pull.

Happy Motoring, Mark

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DrDKW