You cannot simply replace the video card. It is soldered into the laptop's motherboard. You need to bring it into a repair shop to have the video card replaced.
Laptops video card uses the MXM type format. Size ranges from MXM Type 1 to Type 3. The higher the type, the larger and generally more powerful the video card is... and the more expensive they are.
Here's a listing of some laptop video cards. However, they cannot be installed on just any laptop. Your laptop must be compatible with the card.