in the era of all the systems integrators and OEM's you are, in all honesty, better off buying a pre built one from someone. Back when I first sarted in the computer field and laptops became more popular you could build them. But now with the cost of laptops from places like Dell where you get a warranty and all that I dont even see the reason wanting to build one. PC's are a differen story, I will always build my own PC. I am sure there are places out there that you can get parts, but laptops are so proprietary (sp?) that it may prove to be a more of headache than its worth, and will more than likely cost you more than it would to just buy one from Dell. Thats just my .02 on the idea but I will look around and see if I can dig something up.
The problem is that there aren't really any standards for laptop hardware(aside from user-replaceable parts like hard drives and memory, and sometimes cpus). Most laptop mainboards are going to have just about everything on them; you won't be able to choose your video card, for example.
There are some barebones laptops out there that will let you put in your own cpu, memory, and hard drive, but I don't really see the point in doing that. Your probably won't save much money(if any), and you can already choose those parts when buying a regular laptop anyway.
If you truly want to build your own laptop from the ground up, the only feasible way to do it would be to buy a bunch of oem parts that go together. But all that will get you is a laptop that costs about three times what it would cost to buy one already assembled.