I got one for my daughter from Gander Mountain. They have a bunch of them in stock so you can try them on and they are reasonably priced. Another option (if the CSPC has ruined that avenue) is to find a BMX helmet. There are many of them that are basically the same thing as a MX one.
John
1985 Eddie Sanders ATC330r
1986 Monstecate
1984 ATC70 (awaiting 125cc transplant)
1984 ATC125M
1986 GSXR 750 - Keepin' it old school!
1999 Arctic Cat 500 4x4 plow machine
1999 Arctic Cat Thundercat 1000 (lake lizard)
1996 Arctic Cat Jag 440 L/C (ditch banger)