Kettenberg had some for sale a while ago, but you’d have to check with him and see if he has any more. The Montana Historical Gunmaker’s Guild occasionally has some for sale. I make my own. I could send you the recipe if you’re interested.
According to period literature, this was considered to be an exterior grade varnish, suitable for window sills and other high exposure applications. One source also mentioned that it would withstand boiling water. I’ve never found any sources that specifically said it was being used on guns, but it was apparently a readily accessible high grade varnish, so it would seem inevitable that it ended up on some guns. Kettenburg may have specific references to gun use.