Yep, its a Plastimo fitting, you’ll find it here http://www.plastimo.com/catalogue/index.php?LangID=1&catid=6
look in ‘ventilation’ in the left hand menu, and then ‘ventilator’ – ‘deck ventilator’. It’s part no 13358 for S/S and a decent chandlery should be able to get it for you.
Not sure what boat you have but I have just replaced my 34 plastic mushroom vent which sits behind the mast with a new one from Opal marine …. £18.20 +vat.
Nicro now do a stainless one which can easily be made to fit the existing dome shaped one on the B32. It has solar fan, which is superb, and has a switch, and more importantly the latest model has a shut off valve, which the older models do not.