Why would you want to buy s19 firmware?
The firmware for Antminer s19 is free it won't cost anything if you you download it directly on Bitmain.
If you don't know where to download it then check this link
https://shop.bitmain.com/support/downloador I think you are looking for firmware that can able to overclock? Only Braiins OS and ASIC.to that I know good firmware for s19.