I only started taking piano lessons about four months ago. While I have a few other MIDI files, the ones I play are just what you described: the MIDI files for the book that I'm learning from. And, yes, I had to pay extra for the MIDI files... :)
I would suggest using midifiles from the mutopia project (http://www.mutopiaproject.org/)
This sites is a repository of sheet music typed with lilypond, and for each sheet the corresponding midi file is available. These midifiles are created from the same source which creates the sheet, so the two are identical and generally very cleanly written. I found many Bach files to be ideal for learning purposes.
I have recently tried to play a few Jazz standards that I found around the net. Most of the files I found would not work well with PianoBooster, having channel setups quite different from the standard expected by pianobooster (3+4 on grand piano). Of the few files I have found that have the right channels, I found out that the "right hand" is almost invariably impossible to play with lots of five fingered chords. Has anyone else experienced the same, or have any suggestions of "playable" jazz standard midi files?
Re: Pieces of music that I have enjoyed playing - please add to this list
If there are any midi files that you would like to work with pb then please post a link to the files here so I can look at how to improve pb to work with this files. changing pb to use different channels should be relatively easy (however few midi files have the left right hand split). Another easy change is to allow pb to accept a chord even if you have not played every note in the cord. So for example you could allow a chord to be accepted by just playing a single note in the cord. You can also currently limit the notes you need to play by limiting the range of the keyboard using the Setup/keyboard settings although this could be improved.
Currently bp does not split the music up by "Standard midi file track numbers" but only by "midi patch channel numbers". This needs to be fixed in future versions of PB.
Unfortunately many midi files on the net are produced by advanced musicians who like to show off their playing ability and are not suitable for less skilled musicians to play. The only thing I can think of here (other than the suggestions above) is to edit the midi file using a sequencer and delete the all the difficult notes.
Please remember to post a link to any problem midi files here with suggestions on how PB could made playing these files easier.