Wonder if you couldn't get there by winding a 5V secondary on a 5A variac core? Would be easy to pull a turn or two off the primary to get exactly where you want to be.
NitPick Alert... One could assume that you mean 115.000VRMS 60Hz sine wave input gives 5.000VRMS output into a resistive 100ARMS load. But that's an odd set of numbers. Some applications would have a range of line input voltages and a range of allowable output voltages. And when you get to 100A, things like secondary resistance begin to matter. And if you intended to rectify it and get 100A out, there are may be some peak current and core saturation issues... I guess what I'm saying is that you haven't given enough information to specify a transformer...but maybe it doesn't matter. mike