check your interstate catalog (you better be getting it!) youll see that zildjian halves out the higher end hats. heck, they even markets the k/z pair these days. paiste doesnt list them individually, but advertise thier willingness. sabian doesnt appear to outright, but id be surprised if you couldnt get just one.
i usually use a 15" german ludwig standard paiste top, with a 14" swiss ludwig standard bottom. thats my mixed pair

i also have a riveted 14" top that i sometimes swap with the 15 if i want a brighter sound.