If you grew up in the card catalog era as I did (and I realize some people may need to Google “card catalog”, which I find ironic), you know what a treasure online libraries are. Project Gutenberg has been around for books for a long time. But for music students, the world becomes a bright and sparkly place when you find these:
Imslp.org – the international music score library project
Cpdl.org – the choral public domain library
Mutopiaproject.org – the sheet music wing of Project Gutenberg
In these wonderful repositories you will find music scores in the public domain, available to copy, print, download, all free.
One word of caution: you must abide by the copyright laws of your country. The time scale on which items enter the public domain differs from country to country. You can easily find the information for your country on the web. What may be legal in one place may not be legal in another. Do the right thing.