In the full version of Minecraft, obsidain is usually found in the deep part of caves. Since there are no cave systems in minecraft pe, it's really hard to find it. You just have to keep on mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining and mining until you luckily find obsidian.