This page https://store.arduino.cc/usa/mega-2560-r3 contains an image under the Documentation section which shows digital pins 10 to 13 with labels SS, MOSI, MISO, and SCK.
EDIT: The documentation page has since been updated with the correct pin assignment as discussed below. The image has been removed to stop it from showing up in "mega SPI pinout" searches and continuing to spread wrong information.
However, this page says that those are at pins 50 to 53 https://www.arduino.cc/en/reference/SPI.
Which page is correct?