They're available at most bike shops. To get the right fit, you'll need to try them on. They should fit firmly and stay in place. If your helmet doesn't fit right, it will bounce around while riding and won't protect you in a fall. Some of the fancier models come in aerodynamic shapes.
Most helmets also come with small openings for vents.
These help to keep your head cool. If you're riding in the winter and need to keep your head warm, helmet liners are available. Giro , Tel. Once you try the clip-less pedals, you will never go back. You need to make sure your cycling shoes are compatible with the clip-less pedal system you're using.