These are hard to find. But I've been thinking about selling mine:
http://oldtruck.net/phpBB/index.php (under GM Truck Parts)
Relative to the bumpers, you can buy a Chevrolet bumper (original) and drill the additional holes (for the Dagmars) or just run it as a '59 and fahgetabowtit.
I have to question whether modifying a new bumper would be worthwhile seeing as how the reproduction parts stores state that Chevrolet bumper guards won't work on repop bumpers (ie: the repop bumpers aren't true to the original shape)