I have trawled many guild traders tried to do dungeons cos i read somewhere end bosses "could" drop them.. what is the point to all this if the goal is unachieveable?
I would suspect you'll have a tough time finding the motif on traders since the drop rate on the motif during the anniversary events was never that great. This past year, there were less that made it into the market due to the way they did theme-based weeks for the event, meaning we didn't have a massive flood from people doing crafting dailies on a constant basis. In addition to looking in traders, you might go to some of the major hub cities and just announce you are looking to buy that particular motif in zone chat. Some players keep stashes of motifs in their banks and might sell if they know they have a buyer.
If and when you find it for sale, you will become instantly aware that unless you are getting multiple hundreds of vouchers for crafting it, it won't be worth it when the cost of the motif is added to the cost of the tempers.