Before you fill your aquarium with shark, you should spend time researching the various breeds and decide if such an exotic and long-lived animal is a good fit for you and your home. Finding the creature can be a challenge but it is by far the easiest piece of shark ownership. While there are freshwater breeds referred to as "pet sharks," true sharks are saltwater beings requiring huge (100 gal) meticulously maintained tanks. Before you step into an exotic pet store, understand the commitment you will necessarily be making. A recommended read can be found and purchased at the below Amazon website: Aquarium Sharks & Rays: An Essential Guide to Their Selection, Keeping and Natural History.
Research the various breeds. A smaller breed, like a catshark is better than a large breed like a nurse shark in the limited space of home aquariums. Before you buy, you need to be aware of what is "normal" for the breed you are considering. You cannot determine that you are purchasing a healthy specimen if you don't know what healthy looks like. Examine photos and videos and read, read, read.
Join an aquarium club. Chatting online with like-minded aquarium aficionados will add to your insight and open up doors of possibilities for purchases, trades and the caveats of the hobby.
Visit a breeder. While where you live may limit your choices, seeing the condition of the fish and its habitat is an important pre-purchase step. Plan to spend a half-hour in the store on your first visit, without the intent to purchase that day.
Avoid sharks with spots or blemishes on its body.
Look into its eyes to assure they are clean, clear and aware. Their pupils should constrict in bright light.
Check their gills to assess that their rate of respiration is measured and consistent.
Look for activity level consistent with the shark breed.
Set up the aquarium before you bring home your shark.
Buy from a website if your location makes an onsite visit too difficult. The websites below are two options for shark purchase.
There are many exotic fish breeds that are more suitable to aquarium life than sharks. Most fish enthusiasts and ethical hobbyists would shun the idea of entrapping a breed better left to the wilds of the ocean. If you do buy, choose a small breed and a baby shark to reduce its trauma and allow it to more readily to your aquarium environment
If you purchase online beware of their shipping conditions. Some companies make no guarantee that the fish will still be alive when it reaches you. When pet sharks become too large for their tanks, their lives are in danger. Be certain your tank will fit the full life of your breed.