Aside from the ones that are still alive.
Several dead Presidents have never been "buried" below the ground. The following Presidents are interred in sarcophagi, sometimes in mausoleums: Washington, John Adams, John Quincy Adams, Monroe, Taylor, Grant, Garfield, McKinley, Wilson, Eisenhower.
John Tyler died during the Civil War as a resident of Virginia, which some would say was at that time not part of the United States but of the Confederate States of America
If by "United States" you are referring ONLY to the fifty STATES, then Wilson, his grave is in Washington, D.C. is the only one who would qualify.
at 10:17 PM on Jun. 28, 2010