I actually have an R7 240 1gb gddr5 from sapphire in my previous pc.
For windows 10 64 bit: https://support.amd.com/en-us/download/desktop?os=Windows+10+-+64
The main page is: https://support.amd.com/en-us/download
There are only two discrete gpu manufacturers: amd and nvidia. Companies like MSI, Sapphire, Asus etc. are partners which design the coolers, tweak the gpus and sometimes modify the pcb, all of this based on a reference design.