Apple's PPD's are installed by the Mac OS Install program and are stored in the Printer Description Folder (located in the Extensions folder in the System Folder). Older LaserWriter Software installers too install the PPD's; however, the most recent printer software is only available on the current Mac OS Install CD.
Adobe has Apple PPD's posted on their website at:
http://www.adobe.com/
Adobe has also posted many third party PPD files.
Note: Beginning with Mac OS 9, the LaserWriter and LaserWriter Plus PPD's are no longer installed by this Mac OS as no testing was performed with these PPDs and printer.