The king had them cast in clay molds in the plain of the Jordan between Succoth1 and Zarethan.
References for 2 Chronicles 4:17
All these things that Solomon made amounted to so much that the weight of the bronze2 was not determined.
References for 2 Chronicles 4:18
Solomon also made all the furnishings that were in God's temple: the golden altar; the tables3 on which was the bread of the Presence;
References for 2 Chronicles 4:19
the lampstands4 of pure gold with their lamps, to burn in front of the inner sanctuary as prescribed;
References for 2 Chronicles 4:20
the gold floral work and lamps and tongs (they were solid gold);
the pure gold wick trimmers, sprinkling bowls, dishes5 and censers;6 and the gold doors of the temple: the inner doors to the Most Holy Place and the doors of the main hall.
References for 2 Chronicles 4:22