you could get a welder/fabricator build one in a jig for about $400.

You could get a flange cut and make one out of a b16? honda header for about $150..

I understand how off the shelf is "important" if you want that, but a $150 "home made header" would allow for more mods, and dyno time... although there are headers available for vauxhalls in europe off ebay that would still come in under $600..