Cable you mean for power or to connect to the laptop? There are not many egpu models to get, and keep in mind that they don't work with all laptops. For the power supply, that depends on what card you will be using, a good quality 350 watt one should be enough for most cards since it's not powering anything else.
In general using an egpu is not a good idea, better to sell your laptop and buy a newer one with the video card you want instead of running a cable from the case to the egpu hardware which may not even work with your system.