Silly question and I'm clearly late to the party but I just purchased Terraforming Mars yesterday. Where can you get a copy of the promo card?
We have only played twice and I got beat both times but this game is very addictive.
The promo card may be included with the game, depending on where you buy it.
Unfortunately, it seems like not all stores have them, or hand them out anyway.
These promos aren't separately available, and usually come with the game itself, so unless you happen to be in Essen right now to stop by the designers, you will have to try finding somebody who owns extra copies.
There are people on this forum who reported they received extra promo copies with their copy of the game (this probably shouldn't happen), but I imagine there wouldn't be enough to send everyone a copy that is searching for one.
So perhaps it's best to contact a shop who has Terraforming Mars and ask them wether they are willing to sell you extra copies of the promo, if they got any.
(Also, I don't know WHICH stores received the promo...)