Well I don't think anyone is selling any, so far as I know, but it doesn't hurt to look on Ebay. If you don't like the motor bike style helmets, then you should look for one of these helmets. None of these resemble that style.
Hayabusa
C.Q.B.
Scout
Recon
Rouge
E.O.D.
E.V.A.
I think that's it, well, for Halo 3 anyway, there are most likely more helmets from Halo Reach that look different, but there's a good place for you to start.