It is off an E36 or M3 model. bmw expansion tank part no. 17111712641
He are a few examples off Ebay:
New, they are a bit pricey, like all BMW parts but they do come up cheaper from time to time on Ebay.
On a RHD car you will need to modify the mounting bracket to move it an inch to the left to clear the brake modulator. The hose connection comes straight out so you have to inventive with silicone tubing and a coupler. The level sensor works back to front as well. It is nc and goes open circuit as the coolant level drops, so I reversed the action through a relay.
I have since moved the relay. I didn't like where it was.