IMSLP (International Music Score Library Project) is a website dedicated to providing musicians with free sheet music of pieces that fall under the public domain. Here you can find a large library of works ranging from 50 years ago and earlier that are no longer under copyright. The entire project is community driven so all the uploaded scores vary in terms of the arrangement, instrumentation, and date. You may find many PDF's of the same composition but arranged for different instruments and from different dates.  

Grove/Oxford Music Online is an essential music database for those studying music, it is another resource almost every music student will use at some point in their study. The link will take you to the Grove homepage where you can access over 52,000 articles regarding music history, theory, and culture. If you want to know more about ANY aspect of music, you can find an article about it on Grove.  Each article is also easy to navigate and gives you the ability to immediately generate a citation for Chicago, MLA, and APA formats. Some articles may be locked though so you will need to sign in from your school or universities student portal.

The WindRep Project is a comprehensive database of music for wind bands that has been contributed to by band directors, professors, students, and music enthusiasts worldwide. This link takes you to the homepage where you can find alphabetical lists of music organized by composer, ensemble type, genre/style, and instrument soloist. Each entry features the title of the work, the original composer, the various arrangers, the exact length of a performance, the difficulty, instrumentation, publisher, and most importantly, the cost of the piece. For directors or students preparing recitals, each entry also includes program notes from various performances of the piece. 

The Institute for Composer Diversity is one last database that you should have bookmarked in your web browser; This one features an EXTENSIVE database for art songs, choral works, orchestral works, and wind band literature specifically by underrepresented composers. The link takes you to the homepage where you can access all the databases listed above as well as the resources and analysis used by the institute. You can also check out the first of many diversity reports which go into great detail about the kinds of music by underrepresented composers being programmed and how often.