Start with ours, and then move to WorldCat or a large music library with a good catalog. Library catalogs will often give a thorough description and analysis of the score for reprints, changes, etc.
Results in WorldCat can sometimes be overwhelming. Try this older version of the database as an alternative.
Note: Online versions of scores that we subscribe to are included in the library catalog, but you can also search this database directly:
Full, study, piano, and vocal musical scores, of both classic and contemporary pieces.
There are increasingly a ton of scores online. Some advice/warnings:
Heed all their copyright warnings. The only reason they don't get shut down is because of how well it is moderated.
The library half of the GoogleBooks partnership. For scores, search "Catalog" and click "Full-View Only" to limit your results. Unfortunately we are not currently HathiTrust partners, so we can only view and not download items. (Hopefully changing soon!)
There are plenty of decent online music shops, but these tend to have the largest catalogs and best data.
Extensive database of the major European presses with sample pages, good bibliographic data, and a relatively robust stock.
The Amazon of sheet music, carries the largest inventory in the United States. Database is not always up to date with latest ISMN numbers, and search engine tends to sort by popularity, so extra keywords are a must to keep all the "100 Favorite Classics for Piano" from being your top result.
Primarily serves libraries, so has an extensive database of scores, and helpful ways to search, browse, and filter. Does not keep a large inventory, however, so not for a quick purchase.
Primarily used as a way to facilitate rentals and requires a free account to search. Very useful for quickly seeing what is available and the current publisher.