I highly reccomend NOT buying a video card for your laptop.
First off they are much more expensive buying individually then when buying them already built into the laptop.
Second off is heat and power draw. Not sure if your AC adapter is up to providing enough power to a new GPU. Even if it does work - it may make your laptop much hotter (causing unexpected shutdowns).
Thirdly - some laptops have their GPU's Soldered onto the motherboard.
One more issue - for some reason - some laptops willl not recognize newer GPU's even though your laptop may be compatible with a newer GPU.
But if you really want to try it out go ahead. I tried doing a quick search for you for that mobility part - but shops that sell Mobile GPU's are very few and far between and I didn't find any.
Actually just found one: http://rjtech.com/shop/index.php
Not sure if they actually sell mobile GPU's but from another link I searched on google for said they do. You might want to call them/email them and ask them.