Buttons of the time were made of Horn, Jet, Metal, Mother-of-Pearl, 'Composition' and Hard Rubber.
There are folks who collect the things and there are a myriad of varieties.
Your 'plastic' buttons most likely look like what the 'composition' buttons did.
They'll work - just dull their finish if you can with a bit of steel wool.
If you're looking to replace them - look for something in the 'Hard Rubber' range - and Black Plastic will suffice, once dulled a bit.
Both varieties of button holes were used at the time.
Scouts Out!