The part number is 51718137323 and they are about 60p each at the dealer.
I have tried previously to get cheaper 3rd party parts off ebay but have yet to find one which is a good enough fit for the small cost saving involved.
There are tons of trim clips on ebay, mostly made in China. It's a minefield working out which one is going to do a job. And they mostly disappoint even though some quote a BMW compatible part number.