I wanna build a laptop from scratch

I thought of it too but don't bother. You actually don't save a lot of money. I calculated. Plus they are bulky.

If you still want to you need to choose...
a barebone laptop which is the case and has the motherboard and gpu installed already and maybe an optical drive and obviously the monitor and keyboard.
Than choose the rest of the parts like the cpu, hard drive, ram.

you lose money when you do this

also it's hard to find good barebones out there

not if clients are willing to pay for top shelf products assembled in America by a Top Rated American Technology Company

What's the best source for the barebones? newegg, someone else?

AVADIRECT.com has the barebone kits.
You can buy the parts from newegg.