Alienware 18 question

I bought one of the newer alienware 18 laptops. I love it! It looks nice and it's beast. I want to change the graphics cards in it which I heard could be done. Mine currently has sli 770m and I don't need anything crazy like 980 which would be awesome but I need a little more graphical power. I actually just read the owners manual which shows you how to do it. Not bad at all. I just need to know where I can buy them. I'm kinda new to laptop graphics cards so I'm not sure if it will fit. I don't know if different ones fit in different laptops and not others. In fact I'm pretty clueless on anything laptop since you used to only be able to remove the ram and hdd. So any help would be appreciated.

Matt