Farmina is available at Meiko. They have both an online shop and bricks-and-mortar shops in 18 locations throughout Switzerland.
https://www.meiko.ch/de/standorte
To be sure that the store you chose has the specific product you need in stock, call first.
Be aware that the locations shown by orange markers are ‘Meiko Corners’, not the full store, usually located at a Migros do-It and Garden. These only have limited products available. So perhaps look for one of the green marker locations.
(I order regularly from Meiko online, deliverys usually take 3-4 days.)
ETA:
Here’s the Handler locator from the Farmina website:
https://www.farmina.com/ch/storelocator.html?
I put in Zürich as a trial, and see a number of possibilities, including several vet practices that carry it. Again, call first to verify.