http://www.pettravel.com/immigration/unitedstates.cfm
Puppies and Kittens
US puppy or kitten importPuppies and kittens having reached 3 months of age must be vaccinated for rabies and wait for 30 days before entering the United States.
Unvaccinated puppies younger than 3 months entering the United States with their owner and not intended for resale are permitted entry to the US only if they are pre-approved for home confinement by the Center for Disease Control.
Puppies entering the United States intended for resale* may not enter the United States until fully vaccinated and 6 months of age. This includes puppies being imported from Puerto Rico and all US territories. An exception would be puppies being imported for veterinary treatment that is unavailable in the originating country or puppies being used for research purposes. Puppies will also be required to have all DHLPP shots and an import permit prior to entering. *The term “resale” includes, but is not limited to, any transfer of ownership or control of imported dogs to another person, for more than de minimis consideration. So, if you were purchasing a puppy to import to the US from another country, it must be healthy, fully vaccinated for rabies (not before 3 months of age) and be 6 months of age before entering the country.
As I told you in your last question though, if you've found a cheap French Bulldog then be very very sure of its origins before taking it on. If it has been shipped from Eastern Europe forget it.