Acquire as much information as possible about the composition you want to download. If you know only the title of the composition you want, you may be able to find the composer by using a search engine or doing a YouTube search. Later, when you search a public domain free online classical music database, you'll need as much information as possible to easily access the sheet music you choose to download.
Identify whether the music you want is in the public domain. You must do this by consulting copyright laws in your country. Copyright laws can vary; for example, U.S copyright laws are different from U.K copyright laws. A general rule of thumb is that works registered for copyright before 1923 are in the public domain in the U.S.
Arrangements of public domain works are also subject to copyright laws. For example, a piano reduction of Wagner's orchestral Bridal Chorus arranged in 1930 will probably not yet be public domain. See the Resource section for further information regarding copyright laws in the United States.
Search an online music public domain database suited to the type of music you want to print and play. There are numerous public domain websites devoted to specific genres of classical music. One example is the International Music Score Library Project (IMSLP), which is devoted to all genres of classical music in the public domain. Another example is the Choral Public Domain Library, which is devoted exclusively to vocal and choral music. See the Resource section for links.
Join an online forum if you are having trouble locating the music you want. This method works particularly well if the music is obscure and can't be found on a site like IMSLP. Joining an online forum (such as a Yahoo! forum for example) will connect you with collectors and enthusiasts of all sorts of genres and subgenres of classical music who may have access to the music you need.