My world has lots of civilizations which, due to geography, have little access to the kind of sea travel as a means for trade, exploration, and conquest. I think the people of this world may be driven to explore air travel as an alternative. However, I know that airplanes and balloons are "modern" technology, and tend to use modern synthetic materials that ancient peoples wouldn't have had access to. So my question is which of the materials that ancient peoples had access to would work best for building air vehicles, and could they construct entire fleets this way? What would be the practical limitations on their use of this technology?
I'm working with the assumption that these cultures have basic understandings of the principles of buoyancy, and have at least some access to gasses like helium and hydrogen thru magic/alchemy. I am most interested in the construction of the air vessels themselves, such as the balloon envelope which has to be a fine enough fabric to keep gas from leaking out, or the rigid skeletons of zeppelins and airplanes.