If you are like me, reluctant to post on realm forum or ask in trade chat, you can find a blacksmith who has this plan with the following method (though somewhat time consuming).
1. Go to WoW Armory.
2. Look up any popular guilds on your server.
3. Click the Roster, top-right of the filtering options, there should be a drop-down menu “Achievement Points”, click it, and select “Professions”.
4. Select “Blacksmithing” in the Profession filtering option, and set minimum to 300 for the Skill level filtering option.
Enchanted Thorium Breastplate
Now you have a list of blacksmith who potentially have this plan learned, just need to check their armory manually (the time-consuming part).
When you are checking their profession armory, here is a few things to watch out for.
1. Click “Learned” button! Else all the blacksmith you check will have the plan learned (since “Not Yet Learned” is selected too by default).
2. Type “enchant” in the search bar.
3. If that character has the plan learned, check their character armory by clicking their portrait, scroll down to the bottom, check when the armory was updated.
4. If that character looks pretty active (last update within 3 days or so), add that player to your in-game friend list.
5. Ask them to craft the armor for you when you are ready.
I know it is a lot of steps and tedious, but I prefer to have the ability to choose who to ask, and when to ask. I spent around 1 hour searching through 6 guilds, and found about 20 blacksmiths with the plan, and 9 of them were online within the last 3 days.