Southern Exposure Seed Exchange: https://www.southernexposure.com/
Seed Savers Exchange https://www.seedsavers.org/
Open Source Seed Initiative partners with multiple seed exchanges: https://osseeds.org/
It's actually fascinating - I remember my grandmother scooping out the seeds out of vegetables and drying them, to be used next year.
The villagers also traded them as apparently they had different varieties.