How do you find steel in Minecraft pocket edition on iPad?

There's no steel in the game. If you're looking for the steel to make the Flint and Steel, it is actually just an iron ingot which you can obtain by smelting iron ore.

Iron occurs only around from bedrock to slightly above sea level (layers 1-63). However, almost all of the iron ore will be underground. It composes approximately 1.3% of all blocks on layers 5-40, tapering off above that. On average, there are about 77 iron ores per chunk. Veins can vary in size, but the most common size is 2x2x2. Iron always comes in veins of 4 to 10 unless dirt, gravel, a cave, or another ore replaced some. Each block in the vein which otherwise would be stone has a random chance of being iron ore instead.

There is no steel in the game. I assume you are talking about Iron which can be found around and a bit lower then the coal level.

Sadly Steel doesn't exist in Minecraft on any version. If you're talking about iron then you can find it below sea level, but never above it.