Ohiopyle down by the Yough. There’s always good skipping stones there.

Locally, maybe in Duck Hollow on the mud flats by the Mon River. Or even up near the Highland Park Dam Overlook on the Allegheny, but you might need to bring your own stones.

North Park Lake also has a few good stoney shores. In the summer, the boaters, kayakers, and paddle boarders there might make it a challenge (or fun, depending on your outlook).