I just go and get a white flesh fish that comes in fillets. That's because my local shopping strip has 3 different fishmongers who all tend to sell fish that stares at you, so whatever they have that doesn't need to be scaled is what I buy.

I think the last time I did this I used Ling. The book I got it from says Turbot. I've used whiting. Works with most fish that don't have their own strong flavour.