Graham-
That may be true, but they don't seem to be as common as you suggest. When I tried to find a driver for the PosiDrive screws Hewlett Packard used in my counter, there were none to be found. None of the sales people in any of the various tool or hardware stores knew what I was talking about.
I have recently obtained kits from two sources that include PosiDrive bits. But that is after several years of searching for them!
Fred