Trying on helmets is a problem in these days of Covid-19, bring a balaclava or a head sock if you want to try helmets on in person. Which of course, you do! In Snell M rated helmets (Motorcycle ratings, still legal for autocross), open face helmets have all but disappeared. You can still find SA rated open face pretty reasonably priced, but I do not know if local shops carry any, SA helmets usually are a order online thing. You can also use ECE rated helmets for autocross, which more of the motorcycle helmets are going to instead of Snell ratings, for ECE you need either 22.05 or R22.05 rated.
Matt Leicester