All DLs from the App Store or staged in the hidden folder "/private/var/folders/". Use the "Go to folder" finder menu option to get there.
Once inside the folder, search the folders inside for "com.apple.appstore". You may have to check a couple sub-folders to find it as your system generates the path.
Once there, you will find folders for all the current downloading files. Check the "flyingIcon" file in each folder for a picture of the file in that folder.
When the download has completed, the folder for the file will be deleted.
DO NOT mess with any of these files as you may corrupt the DL, but if you are REALY lucky, you may be able to make a copy of the completed DL for backup before it is delete.
FWIW.
Ta