Stone Brickscome from harvesting the "Living Stone" objects, like these:
What Levels have those?
About 30 level in, you'll find a bunch of "Quarry" levels. These are great for finding Living Stones and Stone Bricks. Quarry 4 is the first of them have Living Stones as common rewards for completing the level. Then you can take those back to Camp and harvest them for Stone Bricks!