Thomas Jefferson bought the state of Missouri in a deal known as the Louisiana Purchase in 1803. The Gateway Arch is on the grounds of the Jefferson National Expansion Memorial. While he never saw it for himself a significant part of Jefferson's life lives on in St. Louis.
At least 900 of his letters are kept in St. Louis. It is one of the largest collections of Jefferson's papers outside of Monticello. Most were written during his presidency. All are part of the collection at the Missouri History Museum.
In fact, the History Museum's main building in Forest Park was built as the very first memorial to Thomas Jefferson. Even before the one in Washington D.C. It sits where the main entrance was to the 1904 World's Fair which celebrated the 100th anniversary of the Louisiana Purchase.