I, uh, generally use chocolate chips! The standard milk, dark, and white chocolate chips are available in most supermarkets. Even my local Costcutter stocks them! The bags are half or a third of the size of American bags, but I generally find that an English bag is enough in my cookies anyway.
If I'm melting the chocolate for a recipe, though, I tend to buy a nice bar of chocolate and break that up to melt rather than use the chips.