You buy them used or you buy the few styles/sizes that are sold by vendors like Ecklers (above), Classic Parts, LMC trucks (for rear fenders), and a very few others.
Restoration Specialties, Inc has a page of GM specialty hardware and it shows several sizes. You have to download their catalog and look through the pages for specialty GM hardware (non on-line catalog - just old school). If you cannot find the page number, please let me know.
