I buy them in the bookstore or online. But usually online, because the bookstore where I always go to, has a huge shelf with mangas. And all unsorted. So I get really confused and forget to buy most I wanted XD
I used to buy my mangas in a local bookstore, then they decided to pretty much give up translating and publishing manga in Sweden, so nowadays I read them online. I contributed a good deal to the manga market while I could, at least.
Depends on if it's in English or German. If it's in English, I order it online, have relatives get it or buy it when I'm in America from a book store. If it's in German, I go to comic shops a lot with friends, so I just get it then.