A simple plugin for Sketch to select all exportable layers, slices, and artboards.
Note that this plugin will not select nested exportable elements (e.g. an exportable layer within an exportable artboard). Only the child exportable element will be selected, not the parent. This seems to be a limitation in Sketch since you can’t even do that manually (tested in Sketch 40.3).
Just download the latest release, and double click SelectExportables.sketchplugin to have Sketch install it for you.