For buildings, try Lava and especially Great Buildings Online - search for "airport" and you'll get four to choose from. Some of the buildings here are very low poly, but it depends what you want to do with them. You may be able to adapt other buildings from their collection.

Alverson Fantasy Art Studio has the urban construction kit, revolving door, a bar and probably lots of other useful stuff for interiors.

Arc's Pinup Studio has an escalator which might pass for airline steps - but will certainly come in handy for your terminal building.