Many sources of online music scores have become available over the internet in recent years. Please be aware that copyright laws vary a great deal in different countries with regard to digitized works; take care to follow the laws that are relevant to your particular situation.
The website "Copyright Term and Public Domain in the United States" can serve as a starting place for determining the copyright situation for the materials you wish to use, but keep in mind that this only applies to the U.S.
Any number of organizations, not-for-profit or for-profit, have started to make music scores available online.