Only way you'll get your hands on a laptop case for experimenting with is to buy a second-hand one or one that's old & really cheap, then strip it out and have fun (not!) getting what you need for it.
I don't know of any company that supplies laptop cases with no guts inside, getting compatible parts for them would be a nightmare. I'd drop the idea mate. Just buy a laptop with the specs you require and save yourself a lot of grief.
By all means build yourself a desktop PC from scratch if you want, but as far as building your own laptop it ain't gonna happen. Why would anyone want to? Buy one and paint it in your own colour scheme if you want it to be unique - - lol.