I'm wondering where I can find the aqua theme, such as scrollbars, buttons etc, as images. Where does OS X store these?
See MacThemes wiki.
Most of the graphics are at
/System/Library/PrivateFrameworks/CoreUI.framework/Versions/A/Resources/. You'll currently need something like ThemePark for editing them. In 10.5 it's possible to encode and decode those files with ArtTools.
Since the question was about 10.5, from MacThemes wiki:
Extras.rsrc and Extras2.rsrc are user interface files that are present on Macs running Leopard which contain pre-Leopard graphics and information concerning text colors. Extras.rsrc is used by PowerPC Macs while Extras2.rsrc is the version for Intel Macs. They are by default, unused in 10.5 aside from scrollbars and the Save as PDF button in the print dialog.
Extras2.rsrc are at