Since this is a legacy patch, you won't find it on official modern launchers like Steam or GOG (which come pre-patched to 1.0006). You can find it on:
Moving from 1.0000 to 1.0001 usually preserves saves, but moving backwards from a higher version will definitely break them. Since this is a legacy patch, you won't
or AP-PRO.ru: The biggest community hubs for S.T.A.L.K.E.R. Since this is a legacy patch
Some "purists" want the game exactly as it was in March 2007 with only the most game-breaking day-one bugs fixed. Since this is a legacy patch, you won't
Back then, patches were region-locked. You need the specific WorldWide , US , or CIS/Digital version of the patch to match your game disk/install.