Hello Moumita, there are many shops in Zürich who will sell individualised curtains and several who will tailor curtains you bought elsewhere. But you can't tell them apart for sure without asking them.

Modesa is in the city center, they say that they do small jobs, too. And they have instructions online, which measurements they need to know.

Also, you might search other shops based on this list. I don't know how much this service usually costs. Hope that helps.