You can buy live sharks from SharkSupply.com and FreeMarine.com. However, shark ownership is tightly regulated at both the state and federal level, so make sure that it is legal for you to buy from these sources.
SharkSupply.com offers live saltwater sharks to public aquariums and private collectors. The site provides large species, including hammerheads. However, as of 2015, the company is very selective about the private collectors it sells to, and requires prospective owners to submit a photograph of and information about their aquarium system before buying a shark. SharkSupply.com captures sharks at sea and does keep them in stock, therefore, customers must make all orders in advance of or during the species' fishing season.
FreeMarine.com offers many species of saltwater shark, including the shovelnose, and several species of cat shark. The species the company offers are not always in stock, so check ahead for availability. Free Marine sells both live animals and eggs. The website offers some species that can grow to several feet in length, and require very large aquariums.
Sharks are subject to myriad state and federal laws that regulate whether or not they can be hunted or captured, who can do it, who can sell them, and who can own them. Therefore, citizens in some states can freely buy sharks from these fish dealers, while others are restricted by law to only buying certain species, or are even banned from owning any. For example, citizens of Nevada cannot own any species of freshwater shark.