Laptop Graphics cards are almost always built into the motherboard or attached like a daughter board, usually they are sized just to fit that location...plus you'd have to dismantle most of the laptop to get in there. Plus the power being supplied to that connection is usually set so that if you got another card that required more/less you wouldn't be able to change it.
Lastly, the only place I could think of that would supply laptop graphics cards would be the scraps from partially refurbished laptops.
Just think of a laptop as typically a one-shot deal (besides RAM) when it comes to upgrading things like the CPU, Graphics/audio, etc. Otherwise, if it can be put on a PCMCIA (PC Card) then you can.