Try looking for nice dilapidated houses. By that I man houses that would be nice if they were not dilapidated. Old style architecture, mostly. They have a lot of corbels. Or you can go to some town fair or estate auction, where they sell everything very very cheap. You will definitely find one.
After you get the pair of corbels you might want to do a little refurbishing on them. And by that I mean a proper sanding and painting. And an eventual professional cleaning in order to remove wood parasites, if any.
Next you’ll need a glass top. The length of the glass top is yours to decide. That will be the actual length of your desk top, so choose your own value. The width of the glass top should be the actual width of the corbels. Because the corbels will be attached to the wall and thus function as the desk’s feet, the glass top will be secure on top of them. So the width of the corbel should match the exact width of the glass top.
I suggest going for a thick glass top. Not only it’s safer for you but it’s also looking better. the corbels are sturdy thick pieces of work, so using a thin glass top on them will look very flimsy and out of proportion.
Next comes the last part: the actual fitting of the two. For securing the corbels to the wall I suggest using some corner brackets. You can fix them in four spots each, two at the top and two at the bottom, which will give a tight and most of all a secure fit. If you’re not a good hand with drilling you’d better ask a friend who is to do it. I actually did. Next put the glass top and secure it in place with anything. Duct tape will do. The gently drill four holes in it. Two holes along the left margin, two along the right, and use a screwdriver to fix it in place on top of the corbels.