I thought there was already speculation that the Constitution Party would be willing to give him the nomination if he doesn't get the GOP one.
Yeah, he may not line up with their views that well (except on immigration), but he would easily get enough votes to give their 2020 nominee federal funding, so it would be a great decision for them.
The Constitution Party has ballot access in a mere 12 states, and it'd be difficult as hell for Trump to add significantly more to that unless he starts trying in 2015. I don't think Donald Trump would be interested in a third party run for President where he's not even on the ballot in (let's be generous) 30 states.
EDIT: From the CP's own website: