It's a funny thing, I had one (or was it two) new one for sale on here a while back. I guess old threads get deleted as there's nothing older on here than August 2019. But anyway I don't remember selling them, but nor can I find them here! I'll have a proper look in the shed tomorrow in daylight and let you know.
I presume you've found the make/model id which is shown in the S31 Parts List in the Tech Library. Maybe they are still available from the manufacturer anyway?
If you can conclude that inability to find them means they have been sold, your shed must be a lot better organised than my garage - stuff that disappears in there usually comes to light a few years later.