The easiest way to do elementals would probably be to tie them to any location or instance of their element in sufficient intensity. So a large enough body of water has a water elemental, large formations of earth or rock an earth elemental, storms and hurricanes are host to air elementals and volcanos and scorched deserts are the domains of fire elementals.

If you also make it possible to bring one into being by creating an artificial environment for it you can get some pretty cool ways to have non/low magic summoning. Imagine an army crafting a huge pyre, taller than a siege tower and soaked in oil and pine sap on the fields before an enemy city then setting it ablaze to call a fire elemental into being to help them raze the city to ash. Building huge burial mounds or stone tombs might create an earth elemental guardian to protect the dead, stuff like that.