Visiting Zagreb in September and need suggestions for walking tour guide
I'd go to the TI on Jelicic Square (main square in the center of Zagreb) and ask there. You can google that as well.
Check Tripadvisor as well, they have a few walking tours listed under things to do.
In 2019, we went on a "Haunted Zagreb" tour that was fun, and very silly. The guide had a "spirit locator" app on her phone. We went to a graveyard at midnight. We found the information about the tour at the TI on the Square.