Der Spiegel are tight with Football Leaks. How the latter get their info, I don't know. Probably hacking, although they've been investigated for blackmail and extortion in the past.
According to recent news, Football Leaks was created by a young Portuguese hacker, who has been identified and ousted in national media a couple of weeks ago. He uses methods like phishing, etc, to get e-mail passwords of football related people.
He started mostly getting leaks from Portuguese clubs, but grew on from there. In an (anonymous at the time) interiew with Der Spiegel he claimed Jorge Mendes had sent private investigators after him, but they failed to track him.
At the moment Benfica has been under fire, due in part to leaks from him. As such he is afraid to return to the country and is hiding away in Europe somewhere. Until recently he was living in Budapest.
He was ousted and front cover of one of the biggest magazines in the country, so he's kind of a big deal right now:
"The pirate that stole Benfica's secrets"
"Has less than 30 years and lives in Eastern Europe"
"He's a computer genius who in his first crime deviated 270.000 euros from Cayman Islands"
"Created Football Leaks with information from Doyen, Real Madrid, Porto and Sporting"
"Is under investigation in Portugal and Spain"