Sinar still sell them as new replacement parts. Contact either the Norwegian Sinar distributor, or go directly to sinar.ch use the contact forms there.
There was a repair technician who lived in the UK, Bill Orford, who was active here on APUG and bought up a lot of spares when Sinar UK shut down their local repairs and service department. I've not seen him active for a while, and his website seems to have gone, so I don't know if he is still active or not. If official channels don't respond, you might try tracking him down.