What is the application? details details details! Do you want to step household line voltage (AC) down to 15 volts? Does it need to be portable (working off batteries)? How much current (total wattage) does your application draw? 240 or 120 AC volts should be easily stepped down with a 10:1 transformer, then a bridge rectifier will flip it to DC, then a big capacitor to smooth it out, then your 15V regulator (if you REALLY want smooth power)... If you just want portable DC-DC conversion, I'd recommend a switch-mode power supply. They are far more efficient and can be purchased or designed to run on a variety of input and output values.