What cards can give an opponent control of an Island so I can use Islandwalk on them?
There is an artifact that slowly turns all lands into Islands: Quicksilver Fountain. There are also a few auras that turn the enchanted land into an Island (or any type, including Island): Convincing Mirage, Lingering Mirage, Nylea's Presence, Phantasmal Terrain, Sea's Claim, and Spreading Seas. There is also a sorcery that turns a land into an Island, Aquitect's Will. Some other cards that can turn lands into any basic land type, including Island, are Mystic Compass, Dream Thrush, Illusionary Terrain, Jinx, Moonbow Illusionist, Reef Shaman, Sea Snidd, Shimmering Mirage, and Tideshaper Mystic.

In addition several creatures turn lands into Islands:


Dreamwinder temporarily turns a land into an Island at the cost of sacrificing an Island. Kukemssa Serpent does the same.
Floodchaser temporarily turns a land into an Island at the cost of a +1/+1 counter.
Sealock Monster permanently turns a land into an Island when it becomes monstrous.
Streambed Aquitects, Tidal Warrior, and Tundra Kavu can both tap to temporarily turn a land into an Island.


There are also cards that you can use to make an opponent control one of your Islands, including Bazaar Trader, Donate, Gauntlets of Chaos, Political Trickery, Shifting Borders, Shifting Loyalties, Sky Swallower, Vedalken Plotter, and Zedruu the Greathearted. You can also use Yavimaya Dryad to search for Breeding Pool or Tropical Island and put it into play under your opponent's control.