Updating GPU

Hello guys!
I have an alienware M18x R2 that i bought 3 years ago.
My dedicated GPU has been dead for a little over a year now.
I am looking to buy a replacement/an upgrade.

Where can i buy those?
I called dell and they said that they don't sell parts and that i need to find an independent seller.

Does anyone have an experience with a trusted seller?
What card model do you recommend?

Thank you!