The propulsion technology for an interstellar mission is available today, there are proposals for probes powered by RTGs or even nuclear reactors and driven by ion engines. Jupiter-Saturn conjunctions that can be exploited for gravity assists happen every 20 years or so: https://en.wikipedia.org/wiki/Interstellar_probe#Proposed_in...
It's entirely a political problem. There is no will to fund such a mission, and what's worse is that in the US funding is approved year by year.
In nuclear engines the fuel and propellant is separate. I keep wondering how much gas a rocket needs to take with it in order to have something to push against. Nuclear fuel is incredibly energy-dense, but the density of the gas used for propulsion in a nuclear rocket can't be much different from what's used in conventional rockets.
The JIMO mission was supposed to have 12 tons of xenon on board, a substantial amount of the yearly production (~ 70 tons). You do wonder how Elon Musk's Starlink madness will affect the xenon price.
The StarLink satellites use krypton thrusters instead of xenon [0]. They need more electic power to run, but produce higher specific impulse [1]. And krypton is cheap.
It's entirely a political problem. There is no will to fund such a mission, and what's worse is that in the US funding is approved year by year.