The "Light Clay can't be bought nor found in the region/shops.
The only way to obtain it is to find a Baltoy or Claydol holding this item. Baltoy can be located on route 111, where as Claydol is located in the "Sky Pillar". To save your time, you should get a Pokemon with Frisk and Theif so you can collect the item, such as Noibat, Gothitelle, etc..
Use this if you prefer not to hack
Hope I helped! :)