Well, your chances of getting a ticket sold by BVB are pretty much 0%.
Other sellers like Eventim only get a very little amount that haven't been sold by BVB. A year (or two, I'm not sure anymore) you could get some tickets from
this one rather easily. Not sure how it is today but doesn't look all that good. Seems like they had no tickets for the first two matches this season but that might be because those are the first Bundesliga matches since May. But it might be worth it to check there every now and then.
The other chances are ebay, which obviously is rather easy but most likely expensive, and just going to the stadium. Like at (probably) any stadium, there'll be some people looking to sell their tickets. Depending on your luck, you might get a ticket for the original price in 5 minutes or not get any ticket at all.