Hi Steve,
Welcome to the Forum !!
The LT1108 is older part.
The Diode D1 should be a 1A Schottky. The capacitor C1 is typically a 100uF to 330uF low ESR electrolytic or polymer capacitor.
There is an LTspice model for the LT1108.
You might be better off using the LT3580.
Link:
http://www.linear.com/product/LT3580This part will switch at a higher switching frequency and be a smaller solution size.
What is your input voltage?
What is the output voltage?
What is the maximum output current?
Regards,
Jay_Diddy_B