Motorcycle helmets are acceptable for drag racing and track days, but be sure to get an "M2005" rated helmet. There are some differences with auto racing helmets, and those are rated as "S2005" or "SA05". A Snell 2000 rated helmet, either auto or motorcycle, is acceptable, but only until 2011.
Motorcycle helmets will likely be cheaper. I bought an M2005 motorcycle helmet for $100 down at Renton Motorcycles. http://www.rentonmotorcycles.com/
Pretty big selection and decent prices.
For a genuine auto helmet, the only place I know is Speedware in Redmond. http://secure.speedwaremotorsports.com/index.html
More expensive though, with the cheapest one being $340. But it is your head, so keep that in mind when looking at prices.
I'd get the lightest helmet your budget will allow. Mine isn't too bad, supposed to be 4lbs but I haven't confirmed that. But that is an extra 4 lbs my neck needs to control. Not as big a concern with drag racing, but on a road course, that is a lot of work. I have the Scorpion EXO-200.