There are barebones kits for sure. Scan.co.uk tends to carry Laptops CPUs, but they don't seem to carry laptop GPUs.
2.5" drives in both SSD and mechanical can be bought anywhere. M.2 and mSATA drives are the same. Laptop memory can be bought pretty much anywhere as well.
What has to almost universally be purchased together are the motherboard, chassis/screen, and fan/heatsinks. Laptops with socketed CPUs and GPUs in the MXM form factor can be upgraded, but only within the thermal restrictions of the platform.