Any fishmonger of repute will descale as well as gut and clean any fish you buy if you ask, this includes supermarkets.
However, descaling is pretty easy to to yourself:
Use an large, sharp knife. Remove the fins. Hold the tail and run the flat of the knife 'the wrong way' down the body of the fish a few times. I find this is best done under cold running water as it washes away the scales as they are removed. Give it a good rinse once you're finished and that's all there is to it.
We have got a strange device for de-scaling fish, but to be honest, a knife is just as good.