Stock Airports are located in the fs9\Scenery\ folder. I'll show you an easy example how to get your airport:
E.G. You want to find Belgrade, Serbia Airport. All you know is that it is in Eastern Europe. So, open Eure subfolder and its Scenery sub-subfolder
Now, you’ll get a whole lotta files, with a lot of numbers. Personally, I don't have a clue how this numbers are generated, but I know what (most) of the letters stand for: first you have ab, it looks to me as some kind of "base", and contains airport layouts in that area. Then you have AP Files (I will not hold you any more, they are your files of interest - airport facilities data). Than there are FL as flattens, HL (I think they are coastlines), HP (water masks, I think), RD are roads, RR are railroads, etc. Now, how do I find which file (set of files) do I need? I personally use small utility called LWMViewer, which can be found at various fs websites, and also, here's the homepage:
http://www.jimkeir.co.uk/FlightSim/LWMViewer.html. It has a great option Open by coords in File menu. So if I know longitude and latitude of my airport, I’ll find file that I need. I have attached Example picture, and as you can see, it shows that my airport is located in ??953160.bgl files (Among the others in same area). So I will decompile AP953160 with bglxml, to get what I want. You can also decompile afcad created files, since they are also in xml format of fs2004. There are few other bgl types you can decompile, but even if you manage to decompile them, you'll get .mdl file with them. A bit long post, but I hope it can help.