If you are after DT then only one person knows and that is the cutter in Inigo Jones as it does not come from one area.
For YL there are plenty known however remember that you are in National Park and will be hard to find owner of the property who will give or sell you the raw material.
Same goes with LI.
There is Welsh slate museum in LLanberis where you can admire history and colours od local slate.
Just to add Snowdonia mountains are high but they are still taken over by mountains of Lake District which are also out of Scotland.