You cannot purchase a baby monkey from an ordinary pet store, as there are various state and federal laws in the United States that prohibit owning a primate as a pet. In some states, however, owning a primate is permissible if the owner has a permit.
To obtain a permit to own a monkey, an owner must apply for and acquire a USDA permit and have the monkey examined and approved by a veterinarian. The owner must also provide proof that the monkey is housed in a suitable environment. Additionally, a pet monkey requires a specialized diet and the owner must provide specific amenities required for the monkey to have a quality life.
Finally, many states also require the owner to have insurance to cover any damages caused by their pet monkey. Though it is very difficult to own a monkey legally as a pet, there are online marketplaces where breeders and sellers may offer baby monkeys for sale. The legality of such purchases is questionable and it is important to research the laws of your particular state prior to purchasing a baby monkey.
In conclusion, it is not possible to buy a baby monkey in a conventional pet store, due to laws prohibiting the ownership of primates as pets. In some states, with a USDA permit and other requirements, it is possible to own a pet monkey legally.
Otherwise, there are some online marketplaces offering baby monkeys for sale, though the legality of such purchases varies from state to state.