I second Frank Caggianio's recommendation to turn your Aperture Library into a referenced library (File > Relocate Original) , and this section of the User Manual explains how to work with referenced original image files:
But just a caution: if you do, what Frank Caggiano suggests, access your originals only for reading or copying them, not for modifying them. If you modify the originals directly in an external editor, Aperture may lose the connection to the original files. Also, you will need to back them up separately, since they will not be included in the vault backups.
I use a number of different software packages for manipulating images, making slide shows, etc.,
The safer way would be to use the Media Browser to access your Aperture images in other applications. For slideshows etc. it might suffice to create high quality previews.
To answer your original question:
I can't find Aperture picture files on the hard disk using the finder search feature!
Aperture stores the originals inside the Aperture Library package in a folder "Masters". This folder has subfoders for each import session, organized by the year, month, day of the import. To view the masters, you can select the library in the Finder, ctrl-click it, and use the command "Show Package contents".
See Terence Devlin's User Tip: How do I access my Photos in Aperture?