You are looking for brand new unit of a device which was discontinued over 2 years ago - not an easy task. Only chance is online and you will have to be very careful and very lucky. I personally have no specific ideas.
Support for Windows 10 Mobile is almost the same as other editions of Windows 10, but the support duration is slightly longer. Each version of Windows 10 Mobile is supported for around 26 months (2 years and 2 months) from the date of release. End of support dates for each version of Windows 10 Mobile:
Windows 10 Mobile version 1511 build 10586 / Initial Release: January 9, 2018
Windows 10 Mobile version 1607 build 14393 / Anniversary Update: October 9, 2018
Windows 10 Mobile version 1703 build 15063 / Creators Update: June 11, 2019
Windows 10 Mobile version 1709 build 15254 / Fall Creators Update: December 10, 2019 *
* Tentative dates, support may be extended. The exact date will be announced within 1 year of these dates.
Lumia Support Lifecycle
Lumia phones are supported for 2 years from the date they are launched. This means they get Feature Updates during those 2 years. Each Feature Update is supported for around 26 months. The last Feature Update that a Lumia phone gets at the end of the 2 years will be supported for around 2 years, so the total support period for a Lumia phone is around 4 years from the date it is launched.
In short, a Lumia phone will get Feature Updates for 2 years and it will get Cumulative Updates for the last Feature Update for 2 more years.
For the 950XL OS support ends December 10, 2019. Device warranty is generally 12 months from date of purchase as new, 24 months for the EU market.
Please note that the support from now onward will consist only of bug fixes and security patches. No new features or hardware will be developed further for Windows 10 Mobile. Nobody knows what support will be offered for apps developed by Microsoft.