It is illegal to own a marmoset monkey as a pet worldwide. It is illegal to buy or sell marmoset monkeys in almost all 50 states in the United States. The few exceptions are the 4 states that allow you to have a marmoset monkey as a pet: New Jersey,Texas,Arizona, and Indiana.
You must have a permit and any other necessary legal documents to own a marmoset legally in those states. Therefore, you cannot buy a marmoset monkey anywhere.