Laptop graphics are not readily available from online retailers. More than likely, you'd need to find a broken laptop online, buy that and salvage the graphics card. If you look online, you may find that someone has already done that and is just selling the card.
However, in addition to the expansion slot concerns, you also have to be concerned with the power and heat dissipation. If it requires too much power or does not dissipate heat properly, you could severely hamper or damage the rest of your system.
Generally speaking, not a good idea.
-Wolf sends