You don't. Apple doesn't exactly sell the parts. Service providers have to send back the broken parts when they make repairs. If they don't, they pay a TON more for the part. (Like 8 times as much.)
You will spend a ton more money building a Mac than just buying one, the same way that you'd spend over $100,000 to build a $25,000 Toyota Camry if you made it from dealer parts.
tooki