Tickets for Away-games can only be ordered online on the so called Ticketportal on www.borussia.de. Members of Borussia und of the Fanprojekt Supporters-Club (FPMG) could order first in games with less tickets. System first in first out!
On the other side its still possible to buy tickets personally at the FPMG or Borussia. The FPMG gets always a fix number of enough away-tickets (mostly all the terraces and the cheapest seats). You can even preorder tickets by phone under 02161-177800 or by mail under tickets( at )fanprojekt.de and buy them on the counter of the Fan-House (FanHaus) and the so called Infostand behind the Nordkurve (Block 19) before and after a home-game. If the contingent of tickets is big enough Borussia sells tickets 4 weeks before the game in the two Fan-Shops of Borussia.