If you found a replica helmet that is a little big you could buy a set of pads for a real helmet. They come in different thicknesses so to fit a larger helmet to a smaller head you use a thicker pad. They come in the following thicknesses:
#4 1/2 inch
#6 3/4 inch
#8 1 inch
Here is one example:
http://www.botachtactical.com/msahelmetpads.html