A unitard would be great for the performance. M Stevens is good, and usually more opaque than other weight lycra fabrics. I have many M Stevens tights and unitards and am happy with them.
The alternative is Bal-Togs, who make "custom" stuff. My friends have the best luck with nydancewear.com (get their paper catalogue). I have several white cotton/lycra things from them. I wear these for dance class and socially.
You have to consider the color of your skin, how much hair, etc. and how it might show through the white material. If you want a real "solid" white, you may have to wear two pairs of tights/unitards.
You'll have to experiment with things for the best look that suits you and your body type.
Good luck!
balletboy[/i]