The MiniPCI specification defines a small form factor daughter card for 32bit PCI bus that can be used in notebooks in which standard and small PCI cards cannot be used due to mechanical design constraints. A notebook with such a card can easily be upgraded and serviced. Much more important is the increased flexibility in the assembly of the notebook. The cost for such a card can be reduced by a 3rd party design and production of the MiniPCI card.
As you can see those cards are built into the notebook.
Here you can see an example of a mini PCI Lan Card
It is important to note that MiniPCI Cards will not be available in the after sales market.