Here is another link, I read off Hack a day a while ago: http://emerythacks.blogspot.com/2013/04/connecting-ipad-retina-lcd-to-pc.html

I also bought a few old motorola lapdocks, and one of them I gutted to just a controller and panel. Its only 1366x768, but it accepts HDMI input, and is a good conversation starter at my desk at work when I use it for the occasional 3rd monitor. Other than stuff like that, there is not much that is higher resolution you can just buy.

I have been facing the same problem, and have just accepted the fact I either will have to buy the "smallest" retail display that has at least 1080p (like a 19 or 20"), or custom make something. I have been looking at using an ipad 3/4 LCD connected to a small board and using the displayport on my Macbook air.

I bought a panel off ebay for around $60 USD and am looking at a either a pre-made board at http://dp2retina.rozsnyo.com/ or seeing if someone makes a board for less money. The ipad 3/4 display is eDP so the boards are pretty simple. Then its just making a case for it, which is the hardest part for me!