I don't remember what the document is called, but I think you'd contact these folks?
https://www.stadt-zuerich.ch/sd/de/i...chsene/sz.html
In Kt Bern it was my regional district social help office that gave me a letter/certificate, not the municipality or canton. I wrote to them first to ask for the certificate, based on instructions provided by the municipality. The social help replied with a nice official letter that said they certify they have never had any contact with me. And they included a bill for about 20 Francs.