The better Chinese takeaways around here use the prawns I think you're after, I bought some recently from the freezer section of an Oriental supermarket in Huddersfield.
They were 9 quid a pack which seemed expensive at the time, but by weight they were still better value than the supermarket "king" prawns which are virtually like shrimp sometimes. These ones were about the size of a fat thumb, butterflied and had a very different flavour, really tasty with a nice meaty crunch that I've never had from a supermarket prawn.
Try your nearest Chinese supermarket, I'm sure they'll have them, if not you can probably order them online easily enough nowadays.