What is a substitute for corn syrup in boiled icing?
When heating sugar up in boiled icing or in making candy, the problem is sugar crystallization.  This happens because the solution becomes supersaturated and any movement can cause it to shift back into a crystal state.

The corn syrup is there to prevent this from happening by providing glucose to 'get in the way'.  You can get just 'glucose' at the drugstore to replace the corn syrup.  Any other sugary thing that's high in fructose or glucose should work.

Another option is an acid.  Some acid (like lemon juice) should make some of the sucrose break apart into simpler sugars and prevent crystallization.