for now just walk inside any cycle gear store, try and buy what you like and fits good, but you may wanna upgrade in the future for an actual kart rated top quality helmet like Arai Much better quality but it’s several hundred dollars.
But there’s an old saying, I forgot how it goes? $2 dollar head, $2 dollar helmet?
All kidding aside, Lol. There are a few good inexpensive helmets you can use for karting and they are approved and rated for safety. And you can find them at Cycle gear anywhere from $100 to $600 bucks. Good luck to you.