There are two different types of Elevator Keys. The first one resembles an actual key that is orange in color. The second Elevator Key resembles a Card Key that is blue in color.
The first type of Elevator Key is found in a room of Riverside Station where Mario needs to use his newly required Spring Jump to get to it. The Elevator Key can then be used on the elevator back near the entrance where it leads him down into a room to activate the bridge. However, a bunch of strange creatures were first seen down there.
The second Elevator Key can be found in a room of the X-Naut Fortress. It can be used to access the elevator of the fortress.