Eldritch Stones can only be obtained by the player through Researching and crafting them, or finding them within Monoliths. In the research, Eldritch Stones are classified as Eldrich Knowledge, meaning you must obtain the theory by researching Eldritch Artifacts, followed by researching the resulting Eldritch Fragments.
The researched Eldritch Stone is crafted in increments of four and can be found in chests inside monoliths. It currently has no known uses aside from being a building material. For more information regarding Eldritch Stones and Monoliths, see the Eldritch Monolith page on the official Thaumcraft website.