Black heat shields are doable.
The cheap method is barbecue paint, it's what at least one F1 team uses (or at least used) on the heat shields on their cars!
Getting more spendy is powder coating, I've used Oxford Wheel Repairs on the Horspath Industrial Estate and they are fine, I've encountered some shoddy outfits. Go for a more satin/matt finish, gloss black is a shocker for highlighting otherwise invisible distortions.
Garage 56 don't offer colours because the logistics make it quite costly.
Oh, don't forget to use a dab of anti-seize on the bolt threads (e.g. Coppa-slip), as stainless steel galls without it, and a washer under the bolt head to help avoid the underside of the bolt head tearing the powdercoat.
