Try one of the personal shoppers, I have used one of the Selfridges ones before and she’s come up trumps with Celine for me twice now. Her insta is anika_personalshopperNot yet - but when I do a store availability search it comes up with nothing
The concessions in the stores don’t always put the stock out on display either, I’ve had to ask for something and they’ve brought it out.
Good luck!