I've had good luck with these folks >
http://www.ironhorsehelmets.com/
I'm a firm believer in wearing a DOT approved motorcycle helmet. There is a lot of discussion here on the forum regarding helmets, both pro and con.
Just remember, it isn't always your speed that will dictate if an injury is possible but the actions of others around you. There is also the possibility of an unexpected event such as a mechanical failure that could compromise your safety.
There are several types of helmets and you'll need to decide what is right for you. Half helmets, open front and full face helmets are three of the most common.
Tom