Bent Bike usually has some screamin' deals on helmets.
As for when a helmet is required, this is copied from the NHRA website:
"A helmet is required for the driver of any car or truck running 13.99 seconds or quicker in the quarter mile, and for the rider of any motorcycle. Most drivers can use a helmet meeting SNELL M85, M90, M95, SA85, SA90, SA95 or K98 specifications. Professional category and some alcohol burning vehicles require an SA rated helmet. It is important to note that the helmet rating must be designated on a tag INSIDE the helmet, or sewn to one of the helmet straps. The designation stenciled on the exterior of the helmet is insufficient for NHRA technical inspection. Also, some tracks require a helmet on ALL drivers, so check in advance."
Not that it will effect any Mustang owners, but Russell Stevens also told me that drivers of t-top cars will be required to wear a helmet this year also.