I know I'm a bit late, but Marshadow Event is now active internationally. Just got my card yesterday! It's an in-store event sadly, straight from Serebii:
United States - At Gamestop until October 23rd 2017
Canada - At EBGames until October 23rd 2017
Mexico - At GamePlanet until October 23rd 2017
Australia - At EBGames from the next few days until October 23rd 2017
United Kingdom - At GAME until November 16th 2017
France - At Micromania until November 16th 2017
Germany, Italy, Norway, Sweden - At Gamestop until November 16th 2017
Belgium, The Netherlands - At Intertoys (Code), GameMania, Cora, Smartoys & FNAC (Nintendo Zone) until November 16th 2017
Spain - At GAME (Nintendo Zone) and Amazon (pre-order Ultra Sun/Ultra Moon) until November 16th 2017
Portugal - At FNAC (Nintendo Zone) stores until November 16th 2017
Poland - At Swiat Gier stores until November 16th 2017
Chile - At Microplay until October 23rd 2017
Colombia - At Alkosto until October 23rd 2017
Perú - At Next Level until October 23rd 2017
Panamá - At EstiGames until October 23rd 2017
Costa Rica - At Vértigo until October 23rd 2017
Guatemala - At El Duende until October 23rd 2017
Czech Republic - Purchasing Pokemon-related merchandise (including pre-ordering USUM) in on-line store nintendoshop.cz.
Comes in a Cherish Ball. Level 50. Any Nature. Has Wishing Ribbon.
Moves are Spectral Thief, Close Combat, Force Palm, and Shadow Ball.
Though it's not holding an item, the serial code will also give you a Marshadium Z.