I do not have them, but I believe they might have been at my LFS once! I was curious what they were but I was too late and it was sold weeks before I was able to check it out. I believe it was labeled "Large Blue Shrimp" and the owner described it as being more of a prawn.
As for being common, in my area atleast, I can only claim I have heard of it but never set an eye on it, hah. I also check a few petstores at a nearby city (bi-)monthly and never seen them there either.