Went to Bayern v Werder Bremen in the German cup in October last year. As the crowd was 64000 plus, I managed to get tickets from the club with no problem, but the Dortmund game will be the biggest of the season at the Allianz with a 69900 crowd, so I'd say you'd have to use an agency, a tout, or get lucky before the game off a local.
You can keep checking the Bayern site daily, who use viagogo, which is the same as our seat exchange.com., for any spares. The well known agencies are pretty reliable I believe, but most won't post to England, but will deliver to your hotel in Munich.
If you decide to chance getting tickets outside the ground beforehand, catch the s-bahn or u-bahn from the city centre to Frotmanning station for the ground. There are people selling tickets at the top of the station steps, although how much and how many for this game, I don't know.
If you do get tickets beforehand, don't buy a train ticket to the ground and back, as your match ticket counts as your travel ticket.
Another idea may be to contact the German Villains supporters club and see if anybody there can help out in some way.
I don't know if you've been to any German grounds before, but if you can get tickets, try to go to a Dortmund home game. The South Stand, when it's full is an awesome site.