How can I find free camping locations in Iran

Basically everywhere, as long as you don't pitch your tent where you obviously disturb others. I am honestly not 100% sure about the official rules, but wild camping is very common in Iran.

Just google "wild camping iran" and you will find many stories on the subject, e.g. this one.

Camping is totally free and legal in Iran, and so common and popular. At times of long holidays, summer vacations and nowruz (new year 13-day holidays), you will notice many campers around the country.

That being said, you better search around for places designed for the purpose, since camping is supported by the local authorities, and in special places like parks or schools, people are provided free or low-price services like: tents, free wifi, hot water, electricity and ...

But you need to consider that there are no-camping zones too here and there (e.g. too close to rivers or historical sites) which are marked with signs and you better avoid those places. And there are also places where you should pay for camping which provide more services and are often found at beaches or heavy traffic tourism sites.