The Ivy League look goes in and out of style cyclically, when it’s in you’ll find striped short sleeve button collar shirts at Old Navy and J. Crewe.
Additionally you can always find good striped, button-down long-sleeved shirts at Brooks Brothers. Macys is another good source.
FYI, the founders originally bought long sleeve shirts and had them tailored (3/4 length sleeves as per Nick).
Beware of Temu and basically anything from Asia where you don’t recognize the brand, they tend to be poorly made, and the sizes are never right, are rarely button down collar and don’t tend to have breast pockets.