I have never heard of pre-fab ones, although I imagine they might possibly exist. This is pretty complicated stuff as you need to cut a big hole in your roof and re-support what you have removed (as well as support the new weight of the dormer(s) and do all the complicated roof framing. These are usually built on-site. One of the most basic decisions you will need to make is whether you want the walls of the dormer to end at the roof or continue down to the floor. You probably need a building permit for this. If you were to buy them pre-fab, or construct them on the ground, then you would need to hire a crane to lift them up to the roof and this isn't usually very practical.