How to download original firmware to PowerShot A570 IS?
I'm boldly going to guess: you don't.
CHDK doesn't modify the original firmware in any way, so there's no need for users to download the original firmware. AFAIK Canon has never made available a firmware upgrade to the a570is, therefore the only Canon firmware downloads out there for this camera are firmware dumps for CHDK development. There is no way to flash those into the camera and expect it to work afterwards, meaning they are only useful for CHDK developers, not users.