There are many sources for digital scores, and they are categorized here in a few ways.
Library Collections - Digital score collections made available by libraries and archives. Freely available on the web, these are mostly digital versions of manuscripts and sheet music collections.
Web Resources - Scores made available by organizations not affiliated with a school, library or archives. Most of this is published music that is in the public domain (pre 1923).
Databases - Score collections as part of larger online "e resource" databases.
Music for Sale - Commercial vendors who sell individual songs for a low price. Included here as a last resort since you will have to buy it.
Remember, you have to buy music from these sites.