Yes you can buy the tabard for Darkmoon fair. I have. It's the guy selling stuff that looks like art. Go past all the games on the right side when you enter the fair. Looks like he's selling art but he sells DM Fair stuff.
After looking through the database, I disagree. I doubt there's a DM Faire tabard.
There is a Cloak of the Darkmoon Faire for 90 Darkmoon Faire Tickets sold by Gelvas Grimgate (approximate location 47,64). I believe that may be what Electa7 was referring to. Hope that helps in your search!