Unfortunately I don't have a tip where to buy a touchscreen for the 8s. The number of dealers has also become very scarce. One option would of course be to buy a used working 8s and use it as a spare parts stock.
However, your description of the error has triggered the thought in me that this could be a purely mechanical influence and not a defect of the touchscreen. Such an influence could be caused by dirt, foreign objects, a twisting of the screen or a pressure point in the screen's frame.
In your case, I would demount the 8s, completely remove/expose/examine/clean the screen (if you haven't already tried this) and reconnect the components so that a test of the screen is possible outside the case of the 8s.
It may also be oxidation in a connector or a connector has come loose. For a touchscreen that operates with resistance changes, that would be a possible explanation for the problem.
