Stoddards is the Standards that ALL of the others measure themselves against.
http://www.stoddard.com/shop/shopdisplaycategories.asp?id=23&cat=356+Parts
Separate Catalogs for each Marque.
The Stoddards Catalog is like a reference manual.
I've been using them since 1976 - at one stage they were bought out by Porsche and renamed as Porsche Special Products.
Their prices are reasonable and the service solid.
ALSO:
The US 356 Registry Vendors page is a good starting point:
http://www.porsche356registry.org/resources/356-vendor-links.html
There are some problem Vendors out there but is problematic naming them on a public forum. I guess that you can expect that if they are recommended by the 356 Registry then, they haven't had reason to give them the flick.