No one will have those you will have to build them up.You can call my friend Russ and ask him he might help you out,he;s a Buick nut..510-794-7055
The only 2 Nailhead headers I know of are the shorty's from T/A Performance. These are mostly for Riviera's from 63-66 http://www.taperformance.com/headers.htm and Sanderson's which are for more of street rod thing.http://www.sandersonheaders.com/ though they don't seem to have them on their site. So you may need to have them made. The best flowing of the stock manifolds are for the Nailhead equipped GS's Sometimes you can find them on eBay. Though I'm not sure if they'll fit.
Supposedly the Riviera shorty headers from TA also fit the full size ch***is. They will not fit the midsize Skylark ch***is.