As stated above any laptop with Intel integrate graphics cannot be upgraded. In fact, there are only a very small number of laptops that will allow you to upgrade the graphics card. Those usually cost over $1,500 and you need to research if the model you want to buy will allow you to upgrade the graphics card.
Lastly, laptop video cards (MXM format) are not generally sold to the public. There are only a few stores that would sell them directly to you and you have to research that as well... and they will be expensive. I think I saw a Mobility Radeon HD 5470 that was being bidded for around $175 on ebay sometime ago.