One of the biggest problems in searching for pieces of music is that the titles are tricky. A distinctive title such as Lincoln Portrait is easy enough—but how many works are called Symphony? Thousands! How about variations such as Sinfonia, Symphonie, Sinfonie? What about arrangements of that work for piano four-hands or for wind band?
The uniform title was developed to help with this problem. If you want to know more, please see this tutorial on using preferred titles:
A note on terminology: "sheet music" and "music score" are often used interchangeably, although "scores" can also be used to refer to sheet music that is bound together and contains multiple parts on one page (e.g., vocals and instrumentals all printed together). On this guide, I will primarily refer to "sheet music" as the more generic form.
Music indexes are the best way to find where (and if) a piece of music has been published.
These databases also contain some digital sheet music/scores, but in addition to other document types.