You cannot officially buy one of those Smart cars in the U.S. However, there is a variant of the like on sale in California. I frequently see them in the streets of San Francisco.
Smart car is designed to be used only within city limits. They are not designed to go fast, since the base model only has 41 hp and the more powerful model has a whooping 54 hp. It is virtually driving a moped with doors.
The Smart is not cheap consider what you are getting. They go for close to $10K.