how about the Sun?
if we're talking about an interstellar probe, why not use the Sun for 'gravity assist', ie angular momentum transfer? You could then get a parabolic or hyperbolic trajectory right out of the Solar System. Maybe start with a trip to Jupiter, then hurl it inwards from there on a path which could graze the Sun's gravity well such that it's ejected on some super-fast hyperbolic highway - hopefully pointing in the right direction ..
I don't know if that's viable .. but was wondering about it.