Where can you find sheet music?
Fortunately, with the exception of certain compositions enumerated in Golan v. Holder, anything published before January 1st, 1923 is in the public domain in the United States. No more works will enter the public domain until January 1, 2019. Different rules apply in different countries; please seek qualified legal advice.
The International Music Score Library Project has tens of thousands of public-domain compositions available for download for free.
Mozart’s complete works (searchable) are available for personal use only at the International Mozart Foundation’s web site.
The Choral Public Domain Library has many thousands of free public-domain works as well.
The Internet Archive also has a large number of free public-domain scores.
Some items may be available for the Amazon Kindle or as print books at Dover Publications in their Music Books & Scores. These won’t be free, but you may find some gems here that won’t be available elsewhere. Some are in copyright, but many are in the public domain.
Why should you care that something is in the public domain?
Simply put, if music is in the public domain, you can do anything with it, any time, any where, for profit or not, without any legal consequences. By contrast, if you play a work not in the public domain in pubilc, make a loop from it, upload a recording of it, or anything other than play it for yourself in private, you could have a legal battle ahead of you, which you will probably lose. Now you think this might not apply to you, because you’re not going to make a CD or mp3 file and sell it, but there’s a seemingly innocent way you can get in trouble.
Here’s the scenario: your ten-year-old figures out a popular song, written after January 1st, 1923, by ear. For most of us, that’s every song popular since we were born. His aunt (your sister or sister-in-law) is so impressed she posts a video of your ten-year-old’s performance on her Facebook page. Yes, you can get sued for that.
Buying the sheet music to a song that is copyrighted does not give you the right to perform it; it gives you the right only to learn to perform it. If you don’t understand the difference, get a lawyer versed in intellectual property to explain it to you, or stay away from it.
But what about those performers in various places doing covers of popular songs? Either the venue pays a licensing fee (look for the prominently displayed BMI and ASCAP stickers); in the case of popular tribute bands, the performers may have paid a licensing fee; or they are risking having an A&R representative in the audience and receiving, at best, a cease and desist letter, or worse, a lawsuit. If you think this is excessive, licensing fees for “Happy Birthday,” just recently ruled to be in the public domain, used to be as much as $10,000.