good solo pieces for an alto voice??

These days it's common to find works orginally written for 'soprano' or 'high voice' published (and performed) in transposed scores several notes lower than the original, so in a way the sky is the limit. It's even possible to find scores of the great lieder that were written for high voice now performable by an alto (these days often classified as a mezzo soprano by the way).
When you say 'classical' I assume you include music of the 20th century where you'll find MUCH more written for a lower female voice.
One of the most beautiful and outstanding examples is Britten's 'Phaedra' (Opus 92 I think), which was written for the mezzo/alto Janet Baker. There's a You Tube performance of that...
Anything else I can help you with (I'm a professional composer), write to me on [email protected]

Good Luck,

Edward (Cowie)