You get the formula from any alchemist in Kaslow, Jale or Ilya. In Kaslow, the alchemist(Banight) is located at approx. X 257 Y 265. Talk to him and click on "Take a look at what is being sold". Another window should pop up.
Click on the second formula under the Alchemy Clay on the left hand side of the window. It should cost 8 silver and 40 copper pieces. Upon buying the formula, talk to Banight again and click on "Alchemy in Progress" and place the formula in the top left hand slot under the yellow "Formula" title. Look at the materials to see if you have the appropriate ones(Should be 10 Cenozoic Stones).
Once you have checked all those required, click on "Alchemy" and voila you should now have your resonance stone.
Note: The alchemy may take multiple attempts to succeed.