It was probably this one:
https://www.thefastpark.com/airport-parking/atlanta-atl-hartsfield-jackson
But yeah, it’s on the International Terminal side so isn’t an option for domestic flights IF you need to check a bag. If you are NOT checking a bag, anyone can use the International Terminal even if taking a domestic flight. Many people like doing this as the International Terminal security waits are often far less than on the Domestic Terminal side. But again, that only works if you don’t need to check a bag as you can’t check a domestic flight bag at the International Terminal.
I’m not well-versed in off-site parking options that serve the Domestic Terminal side enough to offer any recommendations, but there are gobs of them around. Hopefully those with better experience will come along to offer advice.