An independent Presidential candidate needs to file 79,939 signatures by 5/9/16 in Texas to get on their ballot. Texas is about 1% of the total national vote.
Of the parties which are already on the state ballots, Libertarian Party is in 48 states (+ DC), Green Party in 36 states (+ DC), and Constitution Party in 26 states (but not DC). That’s not much to work with for a third party or independent run; but, in these days of social media, a write-in candidacy just might go viral given the right circumstances.