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According slide exchange: In UK there used to be a Postal microscopy society that did exactly this. You can check whether they are still active and whether they take members from the underside of the globe.
Microscopists are very different, some like to research certain topics on make their own preparations for this, often fresh. Some build up collections of permanent slides while others just keep micro photos. Some have dozens of microscopes to observe their three different slides again and again. Some exchange slides and material privately. I think the group that would take part in a public exchange with enthusiasm wouldn't be that big.