You can't buy an add on graphics card for an existing laptop, you will have to buy a new laptop.
what if use PCI adapter for laptop to attach desktop graphics card?
There's really nothing that I can think of that would allow you to do that. I've seen a device that converts any GPU to a USB device but I'm not sure what drivers would allow this device to be used.
But that's definitely for the enthusiast. If your system uses a standard MXM slot, you might be able to upgrade the card, but then you need to go through the process of figuring out what works with the system board, do you need to flash vBIOS for a compatible card, etc.