The parts you are talking about are proprietary, and Apple has decided not to provide parts or upgrades. If it breaks, they will replace like-for-like, no upgrades. (So the parts may be in stock, but they are not on any price list and cannot be sold individually).
You also cannot buy a replacement or upgraded graphics card or a fan assembly for that Mac. Apple does not sell these (as parts) either.
If you do not like that policy, I recommend you let Apple know through this link:
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