Every now and then a GPU for an alienware laptop will show up on Ebay.
It is certainly possible to "upgrade" the GPU in a select few laptops, though most will have it built into the board, it won't be a slot card at all.
The tricky thing here is that any I have ever seen on e-bay sell for three or four times what an equivalent desktop part would go for. I get the impression the only folks that sell them are those replacing an 'old' laptop that is in some way broken enough not to be worth selling as a complete system... thus they are rare to find.
It is not a good idea to plan a laptop system around upgrade ability, if it is possible at all (would require a lot of research before you buy) it will cost a fortune.
The best upgrade path for a notebook computer is almost always a new notebook computer.
If you have a very high end gaming laptop there is a decent chance you have the ability to replace the GPU. If not it is highly unlikely. Even if possible a 4670 laptop 'card' will likely cost you more than 200 USD.