Sheet music is crazy expensive, starting at around $4 for a single song. An app that is going to give you them for free is probably violating someones copyrights. That said, as of right now you can get a free app that will get you "hundreds of thousands" of free sheet music songs.
Download hundreds of thousands of free sheet music titles! Save sheet music for easy access later in your library, or print it out directly! All the sheet music is in the public domain, and it's easy to find free sheet music by famous composers like Bach, Beethoven, Chopin, and Mozart. Also includes a metronome and the ability to record audio of yourself playing!
There are a lot of similar apps, but all of those are selling the sheet music as IAPs. This is not likely to last on iTunes for long, but if you grab it now and back it up you should be able to use it indefinitely!
Correction: My reading comprehension failed me as I rushed to get this up on my lunch break. It is just public domain songs, so there is no risk of iTunes pulling it.