if the video card is indeed upgradable then the first place to look is alienware. expect high prices tho, and maybe some complications.

i have a alienware 7700 that also has a removable GPU 6800 Ultra Go. while i could upgrade it, it would also require me to upgrade my mobo as well.

the mobo is/was about $600, and about the same price for the new GPU.

so, assuming you have luck on your side and your mobo can support the new card, i would check with alienware, or ebay.

you could find out who actually makes that laptop, and check them out to see if they sell direct, but i doubt it.
my 7700 is made by Clevo. but if you narrow it down to the actual manufactor, that might be able to get you more hits on your search