I need to upgrade my iMac to El Capitán because I recently bought -by mistake- a magic mouse 2, that won't work in my current Mavericks OS. My iMac can't be upgraded to Sierra because is a 10 years old one. So, I need El Capitan, but I don't seem to be able to find it in the Apple Store. Do yo know where I could get hold of a copy? Thanks in advance.