While both the Lumia 950 and Lumia 950 XL have been available to purchase online, both new Windows 10 Mobile will officially be available to purchase at Microsoft Store's retail locations in the US starting on Thursday, November 26.
Yes, that also happens to be Thanksgiving Day, which means that those stores will likely have some special store hours. Your best bet is to call them ahead of time to find out when they will be open on that day, as it is likely they will open up later or earlier than usual.
The 5.2-inch Lumia 950 has been available via AT&T online for the past few days in the US and it is also supposed to be on sale at the wireless carrier's retail stores in the US starting today as well. The unlocked 5.7-inch Lumia 950 XL went on sale earlier this week at the US Microsoft Store website, but is currently not available to purchase.