Stop the madness!!
Do not buy a helmet at Wal-Mart for crying out loud.
There are basically 2 type sof helmets,
#1. SA rated by the Snell foundation for use in automotive applications, the interior will be fire resistant nomex or pbi.
#2. A Snell rated "M" helmet for use on motorcycles and go-kart applications, esseintially the same quality as the automotive helmet but without the fire resistant liner.
Any helmet that does not have a Snell rating is ****zit, including all of the DOT crap that is out there (some have DOT and Snell ratings).
As far as used helmets go, don't buy one of them either because you don't know if it has been dropped or damaged, and, if the helmet is over 5 years old it needs to be thrown out anyway because the fiberglass (or composite) is starting to get brittle.
Most real racing organizations require a SA helmet less than 5 years old to participate.
http://www.racerwholesale.com/
is a good place to start, like Bill Simpson once said, "If you have a 50 dollar head, buy a 50 dollar helmet". So if you want to save money, do it somewhere else besides on a safety item.