I find, particularly with GdC and GKR helmets, there are ebbs and flows in their availability. As it has been noted there have been a lot more popping up as of late. I watch these helmets closely and 3-4 years ago they were in short supply. There may have been a few offerings but not as many as of late. I also attribute the lack of circulation of these helmets to the fact that many can't be "upgraded". That is to say once a collector has obtained one there is very little chance they will try to go out and get a better example so the limited pool becomes even more limited. I have no doubt that as some of big whale cavalry collectors age and begin to dissolve their collections you will suddenly find a large influx GdC and GKR helmets come to market.
John