Don't know what checking you've already done but don't assume that because of the age, regular parts stores won't have what you need. Check Rock Auto online and your local chain stores.
If you need a used part, try http://www.car-parts.com
I've found that it's usually easier to find the junk yards that have the car you're looking for and then contact them over the phone directly rather than search for a specific part.
Fusick is a good source for reproduction items.
Here is a link to the GM Master Parts catalog. The original GM part number might be useful for cross referencing with other brands.http://wildaboutcarsonline.com/cgi-bin/ ... splayed=50
Here's a good source for NOS parts. Expensive. https://www.oldsobsolete.com/
Go to the section at the bottom of this forum Parts Sources.