That wasn't the question though. He asked for the main assembly, and that is a bare bones laptop with screen, motherboard, chassis, cooling, and (sometimes) battery. Though batteries are really the only easy component to come by.
Direct from manufacturer might be very expensive for a volume of one (if they'll do it). Buying a barebones from a re-seller, still going to cost you. My understanding is that companies like Sager might sell you a Clevo chassis if you ask directly.
I've tried finding MXM GPUs, and what you see most often are used cards coming from people who already have things like Clevo chassis with SLI 780m and upgraded to what would fit. CPUs seem more readily available, but then you have to trust the seller. Unless you have a tray source through a company though, these will be quite pricey as well.
As you say though, sometimes they are specialized and will only accept specific parts. You also have to be careful with TDP, power draw, and cooling. But they are designed to be modular or they couldn't be customized in the first place.
Certainly not as cheap or easy like desktops, but it is there for the enthusiast.