Your picture is not specific enough. I suspect you want is either part number 9 or 12 on the 41/14 BMW parts diagram.
Part number 9 is: 51216754323 (and 51216754324 for the other side)
Part number 12 is available from W&N and other general BMW suppliers (51211813195).
Part 9 is, AFAIK, nla. However, the brilliant people on this forum have found a number of work-arounds.
Here's one old post that provides a solution:
window rubber bits
I suggest you read the whole thread, but your question is addressed on page 2 (near the end). From the pictures in the linked discussion, it looks like it might be possible to buy part 12 and trim it to fit where part 9 goes. Part 12 is widely available so it'd be cheap to give it a try.
Finally, for part numbers and good diagrams, I suggest you try realoem.com (there's are several better alternatives, but I don't recall the urls). Perhaps you already did (it takes some time to dig around and find stuff like this). Once you find the part number, use it here in a search and often you'll find what you need.