I think that most other aeroplanes with anhedral wings were manufactured quite some time ago, and the retro-fit would be more than the value to be gained. There are also very few of these types knocking around, the cost of design, manufacture, testing and certifying would be astronomical and the number of potential sales would make it unviable. The C17 that I mentioned was a recent creation when winglets/sharklets had already been designed, so to create the wing with them already fitted was relatively simple.