If you want the easiest rule to follow, look for any helmet certified as SA2015. While I would normally advise you to get helmets that meet the upcoming SA2020 standard, those will not be available until October of 2020, which is at the end of the season. Still, any SA2015 model should last you quite a long time.
That said, you have two options in helmet styles, open face or closed face. Open face helmets are regarded to be more comfortable than closed face helmets, and you have more visibility. Closed face helmets can allow you to change out visors for sunny days and they have the added benefit of being able to be used at local karting venues.
The choice in helmets are yours, but to take the guess-work out of all of your shopping, I’ve found the most inexpensive SA2015 helmets I could find on Amazon:
G-Force GF1
Small | Medium | Large | X-Large | XX-Large
If you choose to purchase a helmet elsewhere, just make sure they follow the SCCA’s guidelines. You can find those guidelines here.