Hey,
You can make your own shakes by using whey powder, milk, ice, fruit (?Banana), maybe some low fat yougurt and chuck in some oat bran to fill it out a bit more.
The shakes are mainly advertised because after doing resistance stuff you should get protein into you as quickly as possible to help muscle growth - this doesnt have to be a shake though, if you have time things like grilled chicken/turket with some steamed veg is always good.
Gyms like fitness first tend to have cabinets with pre-prepared protein shakes if you wanted to just buy them; although they will be more expensive than doing it yourself.
Yazoo may be a bit to high in sugars to make it an effective after workout shake - probably not high enough in protein either to make it worth your while.
Hope that helps!!