I bought this one a couple of months ago: http://www.amazon.com/gp/product/B0026I8FT2/ref=oh_details_o04_s00_i01

Just keep looking for a helmet that has sizing details in the description. Google search is also your friend - this helmet just seems to be a kids one (it's really inexpensive), so there might be model details available on one of the other equipment sites.