Verified answer

When the anime came out 2 years ago, you could buy the custom made helmet online for actual motorcycle riding. Though because it needed to follow the standard safety design of a regular helmet, the ears needed to be added as a proper support frame. You can't just slap some on because in the event of a crash they could be crushed inside and actually damage your skull. (Hence the reason why current helmets are round....they 'roll off' any impact) Plus the factor of aerodynamics need to take in. So these helmets ran about $200.

You can make your own for cosplay purposes i.e. slapping some ears on. But do not use it for real while driving on a motorcycle.

Durarara Helmet

Celty Helmet

I don't see why not. It is for cosplay purposes, but a helmet is a helmet, even one with cat ears. Probably could be used as a helmet for riding with.