It is illegal to purchase a monkey without a license in most places, and it is not recommended for anyone to keep a monkey as a pet. In some areas, zoos, sanctuaries, and petting zoos are the only places that you can get close to one.
Otherwise, you may be able to apply for a USDA permit that allows you to possess and care for the exotic primate as a pet, but this is not something to be taken lightly. All primates must be allowed to have certain freedoms and live in a healthy and comfortable environment. Otherwise, it is best to leave the monkeys in a safe, supervised area.