Screens, trackpads, keyboards, batteries, and power supplies are totally different between manufacturers. Even two laptops from the same company may have incompatible parts. You can't go to your local computer store and grab a cheap, compatible battery off the shelf. There are literally dozens of different connectors and sizes across all of the different models, even if the basic functionality is the same. Getting replacement parts is only half of the story.
Most notebooks are barely upgradable.
At best you will probably be able to increase your memory and hard drive size. You will never be able to get a nicer keyboard, more sensitive trackpad, faster CD-ROM drive, bigger screen, or faster video card. Occasionally a manufacturer might offer a proprietary internal Zip drive, but that's usually about it. At one point, the laptop manufacturers came together to try and make laptops as expandable as desktops. You can't find a laptop on the market without at least one of these slots.