As for nuclear propulsion being the only way forward, I doubt that, I suspect that chemical will be more cost efficient for a lot of the trips, especially for unmanned cargo trips.
Chemical means, based on carbon products that we've used up till now, would be very rare on remote planets, solar wouldn't work because planets could be various distances from a star they orbit, or could have different atmospheric, or magnetospheric conditions that would make that impractical, wind suffers a similar issue.
The only power source that could be guaranteed to work at the large scale, both in terms of space and time, reliably, and is likely to be available is nuclear power. With your other options being the quantum vacuum energy or anti-matter, but both fall more into the fiction part of sci-fi right now than the science part.
it was just a fun interesting little thought exercise. I'd love to hear if anyone has a criticism of the reading though.
Hydrogen is everywhere by comparison.
The other reasonable option might be solar power near stars, and forwarding the power on to your ships/distant planets using some form of directed energy (lasers, lenses, etc).
I think the idea is to carry a lifetime supply of nuclear fuel. Nuclear rockets aren't necessarily super particular about what propellant they use in conjunction with that fuel, so you can probably easily either directly scoop or otherwise easily refine/distill usable propellants anywhere you can find gas or something that is liquefiable.