All this said, there is a place for open source sheet music for editions and compositions that have passed into public domain. One of those places is The Mutopia Project, where you can find a PDF and a MIDI file all sorts of music, such as the second fugue in The Well Tempered Keyboard.
If you followed that link, you may have noticed that you can also download the LilyPond file. What's that? It's one of the things that makes me so excited about the site. All the music is written in the GNU Project's LilyPond language. It's a little like HTML, where a simple text file can be rendered as a beautiful graphic. Being a programmer and a musician can having it's advantages!
The thing I like about having the text "source" file for the score is that if I know LilyPond, I can change the way the finished output looks. Take our fugue from above. It is very unadorned, which is good if you're not a beginner and you want to interpret the music on your own. (This is something Peters Edition is good about.)
How about custom piano lessons?