Looked on their website under accessories but only had stirrups/leathers & gullet bars.. Perhaps I'll need to email/ring them.. Have googled it as well but nothing coming up.
Have you looked under "billets"? I think I got some from Thorogood several years ago, they were nylon ones. I'll try and remeber to look and see whether I have still got some - they came as a set and I only needed one or two.