Where can I order a replacement screen for a MacBook Pro (Retina, 15-inch, Mid 2014)?
The best place that I have sourced replacement/upgrade parts for my Apple products has been Amazon. A quick search for your part resulted in a number of good sources.

I have used eBay in the past and more often than not there were severe quality/customer service issues.  Buying through Amazon, has been much more efficient and customer service focused.  

Given that Amazon has sites in the US and the EU (including UK after Brexit) you should have no problem sourcing through them.

The repair itself isn't very difficult if you have some basic experience.  ifixit.com has an excellent tutorial.  You won't need to remove the logic board (thankfully) but you will need to disconnect some components, especially the battery.

For this particular model, you will need a pentalobe screwdriver as Apple stopped using the standard Philips screws.