If it has to be the Windows Embedded POSReady version of Windows 7, I suspect you will have to contact VARs dealing with Point of Sale equipment, as I doubt these devices were made widely available in the retail chain.
If you just want a ProBook 450 G3 with Windows 7, they are still available retail in the United States - apparently machines built using downgrade rights from Windows 10 Pro when that was still allowed by Microsoft. Here (click) is one example.
I guess you have good reason to stick with some version of Windows 7 for now. However, for completeness, I will point out that mainstream support for Windows 7 has ended, we're only just over 16 months from the January 2020 end of extended support (security patches) for Windows 7, and we're getting on for two years past the end of sale date for machines with Windows 7 Professional installed (which I guess will mean that any 'new' machine bought with Windows 7 Professional preinstalled now will have been sat post-manufacture for almost two years). These dates may well be different for the Embedded POSReady version of Windows 7 if you can find such a machine for sale, but we are getting close to the point where it is wise to consider migrating away from Windows 7 if possible.
If you do track down a computer with Windows Embedded POSReady 7, that product has extended support until 12 October 2021, but even then the writing is on the wall.
As I said, you have your reasons for what you want. I hope you can find something suitable.
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