Yes mate. They cost an arm and a leg and have to be imported from the US which makes them even more cost prohibitive. I have used the tape on equivalent and they are an excellent bead, but even they work out at a good Â£2+ per metre for the material alone.
All I use now are USG extra wide tape on beads for 90 degree externals, and their beaded flex for splayed angles. It works out about 60p a metre but makes a far superior corner than standard flex tape which is utter shite.