I’ve found that some patterns can only be found on vendors as limited supply items. For example, there’s a goblin merchant that wonders into Southshore with his mechanical bodyguard. I found goblin jumper cable schematics on him one day, along with about three other items, and entirely new stock on him the next day. So inlaid mithril cylinder may be one such item.
As for goblin fuel, if it’s what I think you’re referring to, it’s made by alchemists.