Hi - could you please recommend where I could find/buy cherry/currant/oak/horseradish leaves for marinades/pickling. Ideally we’d prefer to get bushes to dry for the winter. Many thanks!

Oak:
Search for Quercus robur, petraea, or pubescens in the open cadaster/registry. You probably want to contact Grün Zürich (see bottom of the page) if you need more than a couple handfull.

For the rest I'd contact landwirtschaftmitzukunft.ch and ask for advice/directions.
Keep in mind that "Bio" allows the use of poisons, it bans synthetic poisons only.