Not available everywhere yet, but it is in 29 markets, including the home cities of 26 of the NFL's 32 teams. Prices are the same as last year's DirecTV packages.
https://tvanswerman.com/2020/08/14/directv-selling-sunday-ticket-to-non-subscribers/amp/
https://nflst.directv.com/
https://tvanswerman.com/2020/08/14/directv-selling-sunday-ticket-to-non-subscribers/amp/
https://nflst.directv.com/