It is irresponsible and illegal to purchase a baby monkey as a pet. Monkeys should be obtained through a USDA-licensed facility. These facilities may include breeders and sanctuaries, but the most desirable option would be a rescue program.
Local wildlife rescue centers and agencies may also be able to provide opportunities for interaction between people and nonhuman primates, such as educational programs. Contact your local animal shelters to inquire about the availability of baby monkeys.