I remember going thru this sh*t, couldn't source one from any BMW supplier to boot.
Eventually through some research i discovered Mercedes actually use the same Behr(i think) radiators which have the same part, so ended up getting one for around $20 from local Mercedes dealer in Hamilton. +1 Mercedes and f**k you to BMW saying i had to buy a new radiator to get one.
Can't recall the part number but im sure you can find it. Hope this helps.