As a brass band player, the band bought the music, not me. However, for my own purposes and as Guitarhacker said, while learning to play I did buy quite a lot of popular type music with a similarly understanding teacher, playing anything is good practice if playing it correctly.
I then found that with the aid of BIAB, and quite a few other music programs and especially those that can display notation, I discovered the world of free MIDI music available on line, and haven't bought sheet music since.
Besides which, with those programs, I used to write my own rubbish, mainly to deliberately set myself hard stuff in order to get better at playing it.
Just put some chords together, ask BIAB to put a melody to that, then set to a fast tempo, bring up the notation to play from and then try to keep up, as an example.