No kite events in September as far as I know.
As for spots you can try kiting on Baltic sea for example in Karwia or Łeba. In Łeba there is also a big lake with flat water, but water sports are banned there in some months. I think you can kite there in September though.
You can also try kiting in Gdansk. In September tourists are gone so it's safe to kite. There are two spots Sobieszewo Island and beach in Brzeźno district.
There are also spots on western side of Polish coast like Świnoujście or Międzyzdroje, but I can not say much about them.
Anyway most of kite schools are getting closed in mid September, so there are mostly local people on water, so if you don't want to kite alone it might be better to ask here.