If da brim is what you really want, order it on back-order. If you can't wait, to tide you over until it's delivered: buy an appropriate hat, cut the crown out (you can even hem the cut-out area to prevent fraying/make it look better), sew some velcro to the inside, glue some velcro to your helmet. Voila!
It might look like hell, depending on your sewing/crafting skills, but lets be honest - da brim will never be "cool" (in the esthetic sense) anyway, so who cares?