I have never ordered one before but there is a company called
KB Covers them. Might be worth a look,
The WireCutter reviewed them a year back and seemed to like them.
Moshi sells them as clear models but it doesn't appear that they come in anything other than clear.
As was mentioned previously, Apple's official docs encourage users to refrain from using them. This is a result of the Retina Macs having reduced distance between the display and the keyboard and the elimination of the extra glass cover on these models versus older MacBook Pros.