You will need to buy a TB3 eGPU enclosure. These range anywhere from $200 to $500 and typically come without a GPU. There are some more expensive options (like the Aorus Gaming Box with a GTX 1070/1080 or RX 580) that already come with a GPU installed.
My personal recommendations are the Akitio Node Standard/Pro or Sonnet Breakaway Box 350/550/650. These companies are reputable companies in the Thunderbolt hardware realm.
Note that I am not affiliated with any of the companies mentioned above.
Here is a nice comparison of the eGPU enclosures available today: https://egpu.io/external-gpu-buyers-guide-2018/
My personal recommendations are the Akitio Node Standard/Pro or Sonnet Breakaway Box 350/550/650. These companies are reputable companies in the Thunderbolt hardware realm.
Note that I am not affiliated with any of the companies mentioned above.