You can buy an antenna (Off amazon or likely at electronic stores). This will get you basic channels like NBC, CBS, ABC, FOX which have a lot of the games. You don't have to pay anything beyond the original cost of the antenna to get access to these few basic channels.
There is Amazon/Twitch for TNF for the rest of the year. Twitch will be free.
There are online TV services like Sling, HuluTV, YoutubeTV, Fubo and maybe more. Those all include the aforementioned basic channels and ESPN/others. You will want to check what channels you get before buying. These range from 25-100 depending on the packages, I believe with Sling you can get ESPN+basic channels for 25.
Finally there is NFL Sunday Ticket, which should work without having DirectTV I believe. It would be around 300 for the year though.