Alienware m17x R4 GPU fried, need help finding replacement

My M17x wouldn't turn on last week, so I took it to a computer repair center, and they told me my GPU is fried, it was a Nvidia Geforce 675m.  The told me to order a new one, and they would install it for me, after about 3 hours of scouring the internet for help, I could not find any website I trust that sells any GPUs for laptops.  I read a post earlier about being able to either buy strait form alienware, or buy an aftermarket GPU for it, but I couldn't find either one.  Where can I buy either the same GPU, or an upgrade that would still work in my laptop?