Data repo companion to my {espnscrapeR} package.
Note that for much of this data, you can join against nflfastR via Lee Sharpe's nfldata repo
If you're looking for general NFL data, check out Lee's NFL Game Data site.
For nflfastR proper, you can find it on Github.
There's also the rbsdm.com site that has lots of advanced analytics and boxscores.