Most any M-rated helmet (like your snowmobile/ATV) will be completely legal for Autocross as well. The only downside of them is that they are designed to break apart on the initial impact, whereas specific auto racing helmets (like the Snell-rated helmets) are designed to stay together for several impacts, like bouncing off a roll cage 4-5 times.
Don't waste another minute waiting on parts, or anything else. Just get out there and start racing! You will suck at first, and you will be slower than many other cars, but the only thing you will regret is not starting earlier...