Absolutely gorgeous stuff! The combination of the soft flesh and the slightly salty taste -oooohhhh! With hollandaise sauce... My mouth is watering.
It grows on tidal mud flats, so is common around the east anglian coast. 'Professional' fishmongers should be able to get it for you, in season, if asked. I don't think you'll ever get it in a supermarket - too niche.
Probably not much help, but it's often available on Sheringham market - a couple of � for a huge bag. Also by the side of the many roads in north norfolk, as people just go out and pick it for free.