I know of no source for the A350.
I was moderately surprised to learn, however, that there are surprisingly well-modelled drag curves for many modern jet transport aircraft including the Boeing 777 in a book called Aerodynamic Design of Transport Aircraft by Obert. They will in all probability not be 100% accurate, but they work much better than required for your run-of-the-mill back-of-a-napkin calculation. Keep in mind that for any meaningful analysis, you need a fairly accurate engine model, too, and before going into even more detail you need to be sure whether engine nacelle drag is included in the drag curve or the engine model or (worse!!) in both or neither.
Also, if you just need some independent data to innocently play with, google "Piano X 787 sample analysis".