..... or you could re-work the lock so it can use your existing key. My '78 was missing this seat/helmet lock when I got it. I scrounged one off an XS500 (same exact part). My buddy has lost no fewer than 3 of these off his '80 (he never checks the mounting screws, lol). In all cases I was able to re-key the locks so they work with the existing ignition key.
And speaking of those mounting screws, this is one part on my 650 that I find constantly comes loose, don't know why. I've gotten in the habit of checking the mounting screws any time I'm under the seat. I find them loose all the time.