Unfortunately this Portal turret is a one-off prop made by animator Ryan Palser and it’s not available for sale. Otherwise I would be all over it. Ryan creates models of in-game weapons as a break from being an animator for games like Guitar Hero. He’s produced weapons from Fallout and other famous franchises but this turret from the Valve hit Portal is an incredible piece of work.
He’s careful to point out that this is a turret from the original Portal as the designs have changed a bit in Portal 2. Regardless, it’s still amazing.