Take a look at replacementlaptopkeys.com some are the same and some are not as there seems to be different vendors that made this keyboard, so the answer is really depends on your model, here is the link to the apple retina display macbook pro laptop keys, http://www.replacementlaptopkeys.com/ser...-22119/Macbook-Pro-Laptop-Keys/Detail
Yes, there are many sellers on ebay who sell keys, and there is also dedicated sites that sell macbook keys, take a look at www.replacementlaptopkeys.com they have a great selection of apple macbook key, its very easy to replace, done it couple times on my macbook air.
You may.... have a stuck hinge clip? that sounds like its the issue, so do this, take everything OFF and see if JUST pressing the rubber cup works or not, as nothing is holding it down, and then see if the hinge clip moves freely, and check if the metal pins holding the hinge is not too tight, and lastly see if the hinge is broken, if it is broken, you can just buy a new macbook key from places likes www.replacementlaptopkeys.com, so make sure you check all those things, and I hope you don't have to buy a new keyboard, but usually this sounds like a macbook key issue, good luck. http://www.replacementlaptopkeys.com/mac...-key.html