I'm looking for Poland zip-code data (polygons). I have found data in csv on the http://www.geonames.org/ but there are only point data.
Do you know any free source containing this data?
I think to find free layer polygon for Polish postal codes will be very difficult. You can find several companies offering this kind of GIS data, for example.
I'm not aware of any free source of ZIP code shapefile, doesn't mean it is not out there. Like @til_b suggested you can ask for a custom process to try to capture the data you like. I've seen examples in web maps here so it is available but for a fee in most if not all cases. There was a previous GIS SE answer that might provide another outlet to reach your dataset.