There are a couple of smaller professional leagues, the Canadian Football League and the Arena Football League, both of which play variations on the game (Canadian football has been around for a long time, and has a larger field, 3 downs, and 12 players per side, while Arena football has a 50 yard field and 8 players on the field) but ultimately require much the same skillset for success and the vast majority of the players in both leagues are drawn from the NCAA ranks (though the CFL has a requirement for a certain number of Canadian players per team, many of them played college football in the US as the Canadian college football system isn't nearly as strong).
Salaries in these leagues are usually fairly low. I remember Eddie Brown, probably the greatest AFL player in history (and Antonio Brown's father), had to work as an assistant football coach and substitute teacher in the off-season in Albany despite being the highest-paid player in the league, and that was during the height of the league which is nowadays teetering on the brink. The minimum salary in the CFL is $50,000 and average $80,000 - enough to be a full-time job, but the salary cap is just $4.2 million, which means NFL players like Mike Gillislee, Alan Branch, and Patrick Chung earn more than an entire CFL team this year.
There are a handful of players who have made the jump from one of these leagues to the NFL, but it's like one per year at best. Delvin Breaux of the Saints is the most notable CFL transplant in recent years (he also played in the AFL briefly).
Jeff Garcia, Cameron Wake, Joe Horn, Joe Theismann, and Warren Moon all played in the CFL before the NFL. In Moon's case, he was rejected by the NFL at first due to racism that led to teams not wanting a black quarterback. Kurt Warner played in the AFL for the Iowa Barnstormers before heading to the Rams. Fred Jackson played in a sub-AFL indoor league prior to joining the Bills.
For Patriots fans, the most notable player is probably Doug Flutie, who jumped to the CFL after being underappreciated due to his size in the NFL and then came back years later after establishing himself as the greatest player in Canadian football history. But Flutie was a Heisman Trophy winner. David Patten also played alongside Eddie Brown in the AFL and then jumped to the NFL with the Patriots.
There was a brief attempt at a parallel league a couple years back called the UFL that attracted has-beens (Daunte Culpepper, Ahman Green) and never-was (Maurice Clarett, Eric Crouch) but there were a few players that jumped from there to the NFL such as Eric Moore, who spent a year and change with the Patriots. Josh McCown's probably the most notable guy who played in the UFL, Quintin Demps played in the UFL, as well as some kickers like Hauschka, Novak, and Bryant. But the league was run like ****, players didn't get paid what they were owed because of financial difficulties, and the whole thing felt like kind of a huge joke.
All these are exceptions that prove the rule. The majority of guys who are in the AFL thinking they'll play in the NFL one day are chasing dreams. The overwhelming majority of NFL players go from the NCAA ranks to the NFL, perhaps with some time on an NFL practice squad in-between. The other paths are just moonshots on top of moonshots, or in the case of the CFL an entirely different career.
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