I have only played MGS4. Is the Stun Knife available in any other game in the series?
From the 'Blade Weapons' wikia page:
So it first appears in MGS4, however...
So it's available in both MGS4 and Portable Ops Plus
It is available in MGS: Peace Walker (HD)
I presume it is also present in Portable Ops (+) since the engine is the same.