Dark Elixir Drill at Town Hall 16: 3 of them, max level 11. It unlocks at Town Hall 7. Max level per Town Hall, with how many you can build in brackets: TH7 3, TH8 3 (2), TH9 6 (3), TH10 7 (3), TH11 8 (3), TH12 9 (3), TH13 9 (3), TH14 10 (3), TH15 10 (3), TH16 11 (3), TH17 11 (3), TH18 11 (3).
Tap a Town Hall to open this item there. Dashed: not available at that Town Hall yet.