http://www.aliexpress.com/item/high-quality-capacitive-proximity-sensor-LJC18A3-B-Z-AY-18mm-diameter-10mm-detective-distance-DC-6/485043402.html
that should it do. For a brand name you pay 10x more and chinese dealer distribute worldwide. But you need time - some weeks. These sensor will work with 6V - most brands needs more, standard is 24V. But with a small 12V switching supply you can drive sensor and a 5V regulator for Arduino. For adaption from sensor output to Arduino input i would recommand a optocoupler or a reed-relay