These are some of the best vocal music resources I’ve found online:

FREE resources:

Petrucci Music Library

A large, and always growing, catalogue of public domain sheet music, including a lot of vocal material.  Great for classical art songs!

Choral Public Domain Library

Same thing as Petrucci, only specific toward choral music. Great for traditional arrangements of folk songs, chorales, and other public domain songs.

Full online lessons about music theory, coupled with exercises to help you practice and test your knowledge.  Beginning students, try Note Identification – practice finding notes on the staff – quiz yourself!  Once you’re solid on those, start getting into Key Signatures.  Serious students, dig into Ear Training – listen to intervals and identify them, great for self practice if you don’t have a piano!

Paid Resources:

There are many good-quality piano accompaniment tracks to be had here.  Many classical art songs available. Usually about $2 per song, there are also some song packs that come at a slight discount from individual purchase.

Piano accompaniment tracks for a variety of opera arias in all voice types. $3-$5 per aria (depending on which “pack” you buy), or a $40 annual membership gets you access to unlimited downloads.

Classical Vocal Reprints

For hard copies of sheet music.  Extensive catalog of vocal sheet music – opera, classical, musical theatre, and kids music too.  The complete catalogs of major publishers like Hal Leonard, Schott, Boosey and Hawkes, Ricordi, Schirmer.  If it’s been published, Glendower Jones can find it.