Because this is a pre-release "leaked" build, it is not available through official Microsoft channels. Collectors and historians typically use preservation sites:
The official Milestone 1 release of Windows 8 Build 7850 (specifically version 6.1.7850.0.winmain_win8m1.100922-1508
– Some archive communities legally preserve old beta builds for research/historical purposes. Try searching: Windows 8 build 7850 archive.org