I'm not sure if you can flash the stock firmware via CWM as it will remove CWM and your root doing so. Generally have to use Odin to flash a stock rom, which you should be able to download off of Samfirmware.
Go to www.samfirmware.com, Then navigate to Firmware and Files. Select (Type) Smartphone, (OS) Android, (Model) GT-I9300 - Galaxy SIII, (Country) United Kingdom BTU. Then click send. This should show up all Firmwares and sort them by release date.
If you need Odin then it should be available in the Additional Files section below the Firmwares.
Go to www.samfirmware.com, Then navigate to Firmware and Files. Select (Type) Smartphone, (OS) Android, (Model) GT-I9300 - Galaxy SIII, (Country) United Kingdom BTU. Then click send. This should show up all Firmwares and sort them by release date.
If you need Odin then it should be available in the Additional Files section below the Firmwares.