The airport is in Shoreside Vale just as you head in from the lift bridge, or if you take the tunnel it has it's own turn off, just read the signs.
There are 2 import/export garages. The first is in Portland at the docks. As you enter the main entrance to the docks from the road, head left, and take the first left inbetween the 2 buildings. Go through there, and then you see one of those silo things, turn right. You should be facing a garage with a list on the side, and a blue door, that is 1 of the garages.
The other garage is in Shoreside. Going from the Shoreside hideout, go up the hill(the 4 lane windy road), turn left at the intersection at the top, go over the dam bridge(or damn if you think so), and at the intersection hang a right, go up the hill, and at the turn, just keep turning right and you should be facing 2 garages. The one of the left is the one you want, it has the list on it.
Hope that helps.