Early in the series, they discover a second gate under the Antarctic, complete with a DHD. Later, the DHD is shown to function through a rogue team using the second gate. Even later, the Russians acquire the second gate (well, actually, the first, but that's a bit of a story), but use a DHD they got from the Germans at the end of World War II.
So, the SGC should still have the Antarctic DHD somewhere - but there is a crisis with the gate, and Daniel is sent to negotiate for the Russian DHD. Why? What happened to the American DHD?