You can find a free and paid music sheets from one of the following websites.
musescore.com
Share, download and print free sheet music for piano, guitar, flute and more with the world’s largest community of sheet music creators, composers, … This is a site where individuals can contribute their own arrangements. This is awesome because that means you have an ever-growing database of pieces, but it does mean that some of the quality might be in question.
IMSLP
is a library of public domain music. That means that there are no copyright issues to using this music and that you can freely use any of the resources found on this website. They have a massive database: 460,000 musical scores, 17,000 composers, and 54,000 recordings, and they support 28 different languages.
musopen.org
has a library of public domain sheet music and recordings. However, their database isn’t as extensive as IMSLP. They allow you to search by instrument, which makes it easy to find trumpet sheet music or whatever you’re specifically looking for.
8Notes
is organized by instrument and they have everything. When you go into a specific instrument’s section, they further break down their resources into categories like classical sheet music, jazz, rock and pop, difficulty levels (beginners through advanced), and even play-along tracks. If you’re a music teacher, then you’ll enjoy how much popular music you can find on this site.
Mutopia Project
2124 pieces of music – free to download, modify, print, copy, distribute, perform, and record – all in the Public Domain or under Creative Commons licenses, in PDF, MIDI, and editable LilyPond file formats.
BandMusic PDF Library
focuses on wind band music making it a tremendous resource for band directors and band students. As of this writing, their library has just under 3,700 pieces available. Some other features band directors will appreciate are the inclusion of the piece’s grade level, notation of which parts are missing, date, and genre.