I think it was morri who told me before that derasas are hard to come by in the UK, they are easily to find here in US. Similar in requirements to derasas are Squamosa clams and are suppose to be easier to find in the UK.
All clams are availble Maximas are the easiest to get in the UK I occasionaly see Squasmosas rarely see Deresas any reason you are looking specficaly at Derassa`s?