Düsseldorf is probably the best place to go, and I have seen other people on here recommend it. From there you are less than an hour from, Borussia Dortmund, Schalke, Bayer, Mönchengladbach and Köln in the Bundesliga alone. As well as teams like Fortuna Düsseldorf and Duisburg in the lower leagues as well. When I went, I saw Bayer and Duisburg, got tickets on the day for Bayer, but probably got lucky, because I think it sold not long after. Duisburg and other lower league games should be easy enough to get tickets. So should probably try and get tickets in advance, Borussia Dortmund sell out every game, other teams I'm not sure, might be easy to get tickets for games against the smaller teams. You can fly to Düsseldorf easily enough from Scotland. Other option would be Hertha Berlin, I believe tickets are easy enough to get and they only really sell out against Bayern.