You can use helmets with the following ratings - Snell M95/SA95, Snell M98/SA98, or Snell M2000/SA2000 helmets. DOT approval without an accompanying Snell rating is not enough.

Helmets with the M designation are motorcycle helmets, and helmets bearing the SA designation are for automotive sports. The SCCA allows motorcycle helmets in rallycross, but not for road rally stage competition. The SA designation reflects stricter standards regarding impact resistance and fire resistance but is not required for rallycross. Basically Snell 95+ is good enough, regardless of whether it is a motorcycle helmet or better.

I use a Snell 2000-rated Fulmer motorcycle helmet. The rating is important, not the brand. I checked out a few different helmets by different manufacturers and the Fulmer felt best for me, but that's just personal preference.