I was in the same predicament, running the Stoptech 380mm slotted fronts and wanted to match the rears rotor with a 996 caliper. I went with this
e46 M3 ECS Tuning rotor and it worked fine. You could problably also go with the
Stoptech Gas-Slotted Brake Rotors - Rear. I went with the ECS Tuning ones because the rotor is coated to prevent rust and if you're planning to use the original 996 rear pads you'd get a rust ring. You could also run the front 996 pad in the rear to avoid it but never tried it so I can't comment of the validity of the claims.