You can convert save files to transfer them between different devices (MiSTer, flash carts, Retron 5, real carts, etc.) using https://savefileconverter.com/
GB and GBC save files downloaded from the Internet are often in raw/emulator format.
GBA save files downloaded from the Internet are often in:
Save files can be downloaded from places listed here: https://savefileconverter.com/#/download-saves