just use the web browser who have a double "oo" in their name and search for custom made furniture.. there are plenty of them around..
Cheap solution: If you can find the desktop or some sort of wood with the right dimensions (ikea or general hardware) you can buy height ajustable trestles (also from ikea).
If you want a good quality desk you do one of the above though.
I want a desk, very simple design just flat top and 4 legs, but I want it at a specific height, length and width.
Who would make a custom desk? Cabinet maker?