Where can you purchase Notebook Graphics Cards?

The laptops I am looking at cannot be configured with graphics cards but have open graphics slots available. . the only issue I am having is trying to find out where to buy mobile video cards!|||There is 1 standard (more or less) for notebook graphics cards, MXM. However, not often do manufacturers just leave a graphics slot 'open'. Even if the motherboard is capable of accepting a card, the connector and heat sink are not supplied.|||Best Buy