If you want hit big track events, avoid motorcycle shops, as they probably won't carry many (or any) helmets with both Snell SA & M certs. SCCA Solo Autocross is
okay with M cert only, but
big tracks require the SA cert.
You can look up brands with Snell SA2010 certification here:
Helmet Certification Lists. Mine is M2010 certified, but I don't plan on doing any big track events pretty much ever.
You also might find
this site helpful.