It's included in the part 59821021A. That's pretty much the only way you'll get one new. You might be able to find a lock/key set used, but that's probably pretty far fetched. Good luck.
It has pics of a seat lock barrel from a 748. I'm pretty sure it's the same one so you may have better luck finding one seat lock at a better price. If you want to test it out, try removing the seat lock from your Hyper for fitment. Should take no more than a few minutes.