I'm trying (and failing) to sift through the source, attempting to find the image file(s) for the default cursor. Does anyone know where this is?
If I understand correctly, you are looking for the cursor you have in graphic applications.
The cursor is not just an image; there is a whole "cursor theme" involved (it changes when selecting, writing text, etc.!). So the location of the files are theme-dependent.
In my Ubuntu Gnome the default cursor theme (you can check it with
gnome-tweak-tool) is called Adwaita; searching for it with
locate -i adwaita | grep -i cursor
/usr/share/icons/Adwaita/cursors. The files there are not simple graphic files, but X11 cursor files, whose manipulation is explained in this question: Opening cursor files in a graphics editor?
unity-tweak-tool will point you to the correct site if you have Unity.
You can also check this Q&A: How to change mouse cursor and theme?.
In my standard gimp installation in Ubuntu I can simply open it; see:
% gimp left_ptr
as you can see, is a multi-layer image with the cursor for different size. If you just keep the biggest one, export to PNG, you have this: