Jim27:
Anytime you mention the word "shark" in your posts you might as well start associating it with "money" at the same time. Nothing you will do to keep these animals will be cheap.
You can keep saltwater ponds wherever you want. Your yard, your basement, your garage, shed, it doesn't matter. It all depends on you and where you want it. Just because the word "pond" is used doesn't mean its a huge body of water. Most fish ponds are never really more than 1,000 gallons and USUALLY no more than 500. They are also never very deep. Putting that in perspective, your bath tub is probably no more than 55-90 gallons. You could stick a lot of fish in your bath tub. So a 500 gallon pond could hold a hell of a lot of Koi or other pond fish. Ponds can be any size you want them to be, it just depends on how much you want to spend on them. Small ones can be built for about $2000 with all the filters included. Large ones could cost up to $20,000 if you really went all out. Most yard or garden ponds are just dug out of the ground and then lined with a pond liner, similar to a water bed liner. That is the bulk of a garden pond, the liner that is. Mine are built out of concrete and landscape rock. Then I just painted the rock with epoxy pool paint. There are many pond kits available at local garden supply stores and on the internet. TONS of people have them in my hometown and I am sure they all bought theres at a garden store. They are not hard to find.
Jim