I believe Modine was the OE supplier of these, for my 3.0i at least.
From my notes on my car:
BMW Coolant Expansion tank
17-11-7-573-781
3/15/2013 at 158191 miles, PO had dealer install Genuine BMW expansion tank, made by Modine. Has 2012 date stamp, BMW, and Modine logos on it.
This cracked about 2 years ago and I replaced with a Rein, which is working fine, and is functionally identical to the Genuine BMW one it replaced.
