Probably at Mar-a-lago . . . if there is one.
I'm not sure there will be one because Trump isn't big on reading and book learning.
I think there is a good chance he just builds the Trump presidential monument instead of a library. It will likely be a 600 ft tall gold painted statue of himself. Along with a labryrinth detailing his rise from being the poor son of a dirt farmer, to a war hero in Vietnam, where he once beat the shit out of the coward John McCain in a fight because John McCain tried to grab a girl by the pussy and the great honorable Donald Trump would not have a fair maiden assaulted in his presence. To the time killed 500 NVA soldiers with his bare hands, and won the Vietnam war for 'merica, received the metal of honor, to college where he got straight A+'s and was first team All American in Basketball, Soccer, and Football and won the Heisman, would have been drafted into the NFL, NBA, and MLB, but declined to go into business for hismelf and became the world's first and only trillionare, to defeating the evil Hilary Clinton in the presidential race in the largest landslide in the history of the country, to becoming the greatest president in the history of the country.
Why would Trump build a library when he could build the world's largest statue and a labryinth that testifies to his greatness.