How do you export images when sources are missing?
what does "missing" mean? Do you lost your originals? Or do you search for way to export "on the road" without having all your data online?
If you have smart previews, you can use them to export your images.
If you do not have smart previews and lost your originals you can use plungins to recover images from the preview cache. I know that Rob Cole and Jeffrey Friedl created such plugins.
Retrieving data ...