Here you go, scroll down to trim http://buick-parts.com/cat360/..........:TU: Watch E-Bay for hood ornament several listed
You can also try any auto paint supply store. They will have lots of them to choose from. I almost always use the ones that are kinda kidney shaped with a wire spring that folds out to hold it in place and has a threaded stud and nut. You have to get inside access for the nut which is a little harder but they hold the trim solid, much better than any push in type.
Have you tried e-bay? Just type in buick clips (two words) then hit search. There are also some 58 buick rear moulding clips on there now (another 7 hours time left), I don't know if there the same as you need, item number 160137535548. Hope this helps