You can buy the shirt in any BINCO clothing stores. It's called "White T-Shirt" there.
1. Use IMG Tools (or others) to open "Player.img"
2. Search "tshirtwhite.txd"
3. Replace with the one in downloaded archive
4. Rebuild archive
Where to find: BINCO store, named "White T-Shirt".
And on that bombshell, it's time to end. Good day!