There was an outfit in a particular episode. Where can I purchase one just like it?
Most clothes/costumes/outfits are created by the costume department. If any clothes are purchased from a high street store, they are usually tailored and altered to suit their needs. As the costume department is massive, it isn’t viable to document every outfit used. I’m sure they costume department don’t even know anymore! All we can suggest is looking for something similar, locally to you or on the internet.