There is a libertarian case for Bernie Sanders, and it's one sentence long: he is the candidate least likely to order a ground invasion of Syria.
In fact, on foreign policy issues—and civil liberties that pertain to them—Sanders is significantly better than Hillary Clinton or any of the Republicans.
The American Conservative has put together a helpful chart comparing the various candidates' hawkishness. It should come as no surprise that Sanders has the best report card. The rest are a mixed, mostly awful, bag. Two other things of note: 1) Marco Rubio, seen by many on the right as the only candidate who can beat Trump for the nomination, scores significantly worse than Trump, and 2) a Clinton presidency would be an interventionist nightmare of epic proportions.
Progressives tend to focus on Sanders' redistributionist views on domestic issues—and Sanders himself is much, much more comfortable talking about wealth inequality than war—but for libertarians, there is no denying that his relative dovishness is the most admirable thing about him.
Of course, libertarians have received false assurances from anti-war liberals before. Barack Obama, too, was billed as a non-interventionist candidate—supposedly, that was the one redeeming thing about his candidacy. And there were arguably better reasons to believe Obama would actually govern as an anti-war president than there are to believe Sanders would do the same: Obama, unlike Sanders, talked about ending wars. Obama, unlike Sanders, made military de-escalation a major component of his sales pitch to young people. Obama, unlike Sanders, took office at a time when the public was clamoring for the U.S. to withdraw from the Middle East. In other words, why should libertarians expect Sanders to be good on foreign policy, in the face of manifestly weaker incentives? Arguably, they shouldn't.
But even so, Sanders opponents are all hawks, to varying degrees of incoherence. Libertarians know exactly what they would be getting with Clinton, or Rubio, or John Kasich: more of the same horrors.