I believe that it is called "the magic key"...replacements keys can be purchased from lockeduplove.com but a quick check would seem to indicate that they are no longer in business. I have also found that the chinese knock offs for the Nub and Nano and other HT products come with a similar key and can most probably be used with the Holy Trainer. You might have to purchase another HT device but it will come with a similar key.
You can get replacements from Amazon as already posted, but you can get them cheaper from wish, AliExpress, dhgate, eBay ect. Just search for chastity lock and they will come up.