Mega2560 Pro Mini

Where to buy?

You can buy it here: https://electronics.semaf.at/Mega2560-Pro-Mini

Install to Arduino IDE

Download pins_arduino.h file to folder:
32-bit Machine: C:\Program Files\Arduino\hardware\arduino\avr\variants
64-bit Machine: C:\Program Files (x86)\Arduino\hardware\arduino\avr\variants

Edit the file boards.txt
32-bit machine: C:\Program Files\Arduino\hardware\arduino\avr\
32-bit machine: C:\Program Files (x86)\Arduino\hardware\arduino\avr\
add the content of the file boards-atmega2560.txt to the end of boards.txt and save.

Now start Arduino IDE.