BGG has its own little store where it sells BGG swag, mostly tees from old BGG.cons. It also has a number of “BGG only” promos. 
The most well-known are the Dominion promos. However, they’ve also got a BGG-themed promo for Fluxx, as well as promos for 7 Wonders, Ascension, Stronghold, Survive, Masters Of Venice, and Thunderstone.
You should soon be able to buy the “BGG Power Grid” promo. The expansion will contain several of the previous promos, as well as the new card Theme Park.
Theme Park is mixed into the plant deck and auctioned immediately when it is flipped up, regardless of what phase it is. The card does not count as one your plants; it instead counts as an exclusive city (simply put a house on the card and bump your counter up on the city track). If you’ve played the game, you know that this is a double edged sword.
 These are said to be BGG only, but in practice FLGSs can usually obtain copies if they apply to the publishers.