The iPhone X is quite hard to come by right now, even by Apple's absurd standards. If you buy online you're going to have to wait three to four weeks before it arrives, but if you must get it in person, Apple now has an online stock checker for you.
As noticed by MacRumours, all you need to do is head over to the Apple website and declare your intention of buying an iPhone X. When it gets to the point where you're asked to pick your capacity, click the link underneath that says 'Check Availability'.
Then enter your postcode, and you'll be able to see availability at your nearest 11+ Apple stores.
Obviously this is only for Apple Stores, not any old phone shop, but if a model is in stock (which seems unlikely at the moment, but what the hell) you can opt to reserve it online. That way you don't have to rush to the store in question, only to find that someone else nicked it five minutes before you arrived.
I've checked a few postcodes for big cities, and I didn't get a single hit. So your chances of getting one in store are quite slim at the moment. If you're desperate to get one quickly I suggest ordering and being patient, but if you really must get it in store for that authentic Apple Store experience (*shrug*) then you've now got a way to minimise time wastage. [MacRumours]