Where to find old MTB parts?

I have an old Peugeot Orient Express, a mountain bike I purchased in the
mid-80's. It has Shimano Exage SIS components. "Light Action"

I need to replace my thumb shifters and brake levers. They are cracked and
broken.

Where would I find such parts for an old bike?

On line or otherwise... Any ideas, or links, please send suggestions.
Thanks!


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