I wouldn't know where to look there either. I'm hoping that some of the people that commented on the other threads that use them will see this question and provide some insight.
The search term for all kinds of paperwork, writing and painting utensils would be "Papeterie", there's still a few of them around in Zurich... good luck!
My husband bought his most recent one in Manor. They have a whole range of styles and prices and I think you should be able to find something in the 100 chf range.
I recently bought a Pilot Fountain Pen (recommended by the blog) from Amazon Germany. Took a couple of weeks to get here but I could not be happier with the purchase.
Check out the local fairs. At the Comptoir Suisse in Lausanne, I came across quaint (and excellent) handmade fountain pens in the range of 40-120 CHF. Obviously, these are not brands like Cross or Mont Blanc, but are made by local artisans and uniqueness personified.
Waser has a very good Papeterie in the Old Town of Zurich as well. I ́ve been there for other stuff, and they are mostly very knowledgable.A little further down the same pedestrian street (walking towards the Grossmünster), there is an even fancier Papeterie, sort of nordic design, they might also have better quality fountain pens.
Ok, seriously guys: There is no way to get a Caran D'Ache or Mont Blanc for 100 CHF. Not even close. You will get either some fancy looking ones most likely made in Asia in the big shops like Manor or some less fancy but European looking standard issue school fountain pens.
There are also tons of pretentious looking pens carrying European sounding names but are made in China. It does not matter so much for a biro but especially a cheaply made fountain pen just does not work - it will either scratch or spill (or both).
Only exception I can think of is Faber Castell which makes decent looking fountain pens in the 75 EUR range.