Last year, it was the players association. This year, it could be the referees association...or not. According to the Washington Post, the NFL Referees Association and the NFL are heading to federal mediation. Unlike last year, in which the lockout would have prevented the season from being played, the league would simply find alternative referees is an agreement is not reached in time:

The current deal expires at the end of this month and the NFL has instructed its scouting department for officials to begin preparing for the possibility that replacement officials could be used during the upcoming season if a deal is not struck.

According to the report, both sides are "intent on reaching a deal," so this will probably end up being a non-issue when everything is all said and done. In the event that things don't get resolved, though, where would the league look for replacement officials? In a report from earlier this month by ESPN's Adam Schefter, he explains a possible scenario:

If a new deal is not struck, the NFL would consider locking out officials and hiring college and semi-pro officials, sources confirmed. Those hires would not come from the BCS conferences because the officials in those games are run by NFL officials, and the league doesn't want to put the BCS officials in an awkward situation. The NFL could also turn to a few elite NCAA officials who have retired.

It's not like the league would need just a few officials as replacements, though -- there's a whole slew of guys on the field for one football game, let alone sixteen in a single week.