Castrol TWS is $12.50 at
getbmwparts.com (Tischer BMW). They say the MSRP is $15.63 in one place and $12.60 in another.
BMW part # 07510009420
Bavarian Autosport also has it for $12.95.
Locally, you'll only get it at a BMW dealer or a third party BMW repair shop (they will likely have sourced it from a local BMW dealer).