Finding the sheet music for a particular song online can be a bit like finding a needle in a haystack. Googling the song title and "sheet music" can work, but lots of the sheet music is stored in databases that Google can't search.
Digital collections of sheet music online are good sources for locating historic popular songs (usually composed prior to 1925 or so).
The Music and Performing Arts Library has an extensive collection of historic sheet music separate from the modern printed music editions found in our online library catalog.
Didn't find it? Search the printed indexes to the Sheet Music Collection
These are printed indexes to the collection, organized by year of copyright; you must know the year to search these indexes.
There are many other digital collections of sheet music as well as free and commercial download sites. This list is by no means exhaustive but, if you're really interested in finding more projects like these, a little digging around within the following websites can yield good results.