Most laptops do not allow you to upgrade the graphics card. Only certain gaming laptops which cost at least $1,500+ will allow you to do so. I believe most Alienware laptops allow you to upgrade the graphics card, but you need to call them about that.
Laptop graphic cards are not sold in the retail market. They are only sold to system integrators (Dell, HP, Acer, etc.), however it is possible to buy new and used laptop graphic cards on ebay. Here's a GTX 580m selling for $570. Yes, it does cost more than it's desktop counterpart and it is less powerful.http://www.ebay.com/itm/Alienware-M18x-M17x-Nvidia-GeForce-GTX-580M-3MF8R-W-Br-/190675838486?pt=PCC_Video_TV_Cards&hash=item2c652a6616
Graphic cards for laptops follow the MXM format ranging from Type I to Type III. Weak cards tends to the smaller Type I format while powerful cards like the GTX 580m uses the larger MXM Type III format.