Just found this one of
the Bockengut in Horgen, which is not far from Thalwil. In Wädenswil there is also a 'Farm Shop' (Rellstab I think) on the Speerstrasse. I don't think you can pick fruit yourself there though.
And
this Site was on an Ad on Swiss TV! Again, it is by no means complete, but we'll get there in the end.
Googling 'Direkt ab Hof Gemüse' (straight from the farm, vergetables) gives a few more places but
this one south of Berne is the only link I can find to pick the fruit yourself.