Southern magnolia trees are noted to live up to 120 years in the right climatic environments. The magnolia tree at the White House is called a Jackson Magnolia; Andrew Jackson planted it in memory of his wife, who had died within days of his election (1828). The story is that Rachel Jackson wanted to plant a sprig of a tree growing on their Tennessee farm. In 2017, arborists recommended the removal of the nearly 200-year-old dying tree; saying it would have been removed a long time ago if it was not "historic." The upside is that groundskeepers at the White House kept offshoots of the tree, which are growing in a greenhouse. The "baby" is to be planted at the White House.
To the best of my knowledge, seedlings of the original Jackson Magnolia are not available to the public. You might inquire with your local congressional representative's office.