What did the LORD tell Jeremiah to buy from a potter and shatter in front of the elders?

Answer: A clay jar

Jeremiah shattered a potter's clay jar (KJV: earthen bottle) as a sign that God would shatter the nation and the city beyond repair.

“This is what the LORD says: “Go and buy a clay jar from a potter. Take some of the elders of the people and leaders of the priests,”— Jeremiah 19:1 (BSB)