You can buy switchblades/automatic knives as well as those "gun" knives that actually shoot the blade out from various means (co2,e tc..) at any local gun show.
The military can possess them, though the only rank I know of is paratroopers. They often wear them concealed around their neck/shoulder area. Reason for this is they can access it with one hand should they ever need to cut through their chute chords (hanging from trees, etc..)