Well - first I would suggest figuring out the kind of shark you interested in. This will give a real clue as to what kind & size of shark pond you'll need.
For active swimming species - small requiems - i.e. Blacktip Reef, or Bonnethead. A large circular or oval shaped pond - such as a large above ground pool, or fiberglass aquaculture tank, that are 10's of thousands of gallons. Although with Blacktip Reefs - I would suggest looking for one that at least 5' deep.
For Benthic species - such as bamboos, epaulettes, coral cats & Horns. They can easily be kept & possible breed in much smaller ponds. I've heard of Eppies breeding in a 1,000 gal pond. You can D.I.Y. out of wood & (fish-safe)plastic liners, or you pre-made poly-ponds(like Rubbermaid), or evem small swimming pools, or fiberglass tanks.