Does anyone know where I could find at least some of Perry's letters to Truman?

I'm writing a major on Perry Smith for University and it's very difficult to find these letters. I managed to find two of them online, but I was looking for more. I know that Truman kept all of Dick and Perry's letters in a box of souvenirs from Kansas, but I don't know what happened to it when he died. I was hoping some of them were accessible to the public. Anyone could help me, please?