From memory, they're discontinued...
Had a friend who runs it as estate tweeds and had a similar issue trying to kit out his new under keeper last winter... He did have a suggestion of a website called "toolforfishing.com" - but this was quickly debunked as a scam site, so be warned. Nowhere seems to have any old stock lying about.
Possibly best bet would be asking Lovat Mills in Hawick if they were the ones to make the tweed for Schoffel for them. If so, they'll be highly likely to still make the actual tweed, and you could get a tailor to make you up a set (probably not far off what a Schoffel set would cost you either...).