There was a place online that sold them, but I can't recall where. I made mine from a piece of square steel plate the same size as the flange plate. I drilled 4 large holes to match the header/sidepipe holes and clamped the steel in the saw so it would cut the steel block on a taper. I think mine went from about 1/16" to 3/16" on the opposite corner of the plate. I cut one for each side of the car because one needed to be angled down... the other needed to be angled up. When I was done, the sidepipes were very close to being the same height when they come through the cutouts in the body.
Mike
*UPDATE* I found the wedge shims at Breeze Automotive. They may be Factory Five specific depending on the hole pattern in the shims. Mine weren't as fancy as these, but they worked perfectly and I just happened to have a piece of steel stock the right size. Here's the website for the Breeze shims...
http://www.breezeautomotive.com/search.php?search=wedge