All Windows 10 Mobile Lumia handsets are back in stock in the US Microsoft Store today, including the Lumia 550, unlocked 950, AT&T 950, and 950 XL.
You'll notice that the Lumia 650 is missing from that list, and that's because Microsoft removed the listing for that phone last week, along with the 735. If that was the one that you had your eye on, you might be able to get one on Cricket Wireless if you're vigilant. The listing on Cricket's website seems to come and go, although the AT&T-owned carrier still sells the Lumia 635 and 640.
Indeed, over the last week or so, Lumias seem to be disappearing from various first-party retailers on the web rapidly. Microsoft also removed its listing for the AT&T Lumia 640 XL, and AT&T is out of stock on the 950. The unlocked 640 XL remains on Microsoft's Store, but it's been sold out for some time.
Over in the UK, Lumia fans have it even tougher. All Microsoft-made handsets have been out of stock since mid-December. It would seem that if you want one of the existing Lumias in the US, your time is running out, and if that's the case, you can find them at the following links: