Where can I find Obsidian in the Pocket Edition?
In the Pocket Edition, lava is incredibly rare, and natural obsidian is thus even rarer. Many seeds will not have natural obsidian, and some may not even have any significant amounts of lava.
Apparently, one seed that will work is "herobrine".
You can get obsidian by making the nether reactor. When the nether is over the nether structure will turn into obsidian.
Obsidian only appears when lava source block touches water. Obsidian never appears as part of world generation. Only on the coincidence of a underground water lake.