Since the kits aren't available (I make copies now), I just cut up the scraps from the access holes in the B-pillars to make my own reinforcements. Their shape isn't critical since there's so much variation in the exact positions of the panels during factory assembly, and the amount of bending (which causes the cracks), and the amount of foam sealer, seam sealer, & weld. So some adjustment to the reinforcements is inevitable, anyway, and the rest will be filled in by the adhesive.
To buy a set of patches (with or without rivets), email me through my profile here.