My company is a UK OEM and I assemble laptops (not promoting my company here, just general advice!), You want to Google ODM (Original Design Manufacturer) and try to find resellers of the chassis.
It is unlikely as a individual that you will be able to buy direct, however if you find out the chassis and model numbers, you can Google and try to find a reseller.
Typically, you buy the chassis that has the screen, graphics, motherboard, keyboard and mouse in it and you just assemble cpu, memory, optical, one or two extension cards (Such as wireless) and the software.
That being said, I can tell you that it when comparing to off the shelf laptops, it is near impossible to beat the low end models, some are actually sold below cost and subsidised by the software that is included, however on the very high end models, you can usually save a lot (up to, or over 50%)
A downside though is support. Typically on the laptops I sell, it is very hard to get driver support from the manufacturers, until Nvidia released the GPU drivers for everyone, I was stuck with the version that the manufacturer sent with the model which was annoying... there are some other downsides, but typically you learn to work around them.