Stomata may be found on any part of the leaf, or not at all. This depends on the species.
The primary function of a stoma is to 'inhale' carbondioxide from the air, which the plant will use in photosynthesis.
In a normal plant, stomata are usually on the lower side of the leaf, trees like pine have stomata all around their needles. But if the leaf is floating (e.g. water lily) the stomata will be on the top.
Leaves that are completely submerged, may not have stomata at all (because of them being useless).
From the denisity of stomata seen in fossil leaves, we can say something about the carbondioxide-content of our earlier atmosphere (more #CO_2->#fewer stomata necessary)