Zippo342,
You can find literally tons of iron in just a few asteriods. If current science is correct, there is plenty of these to find everywhere, including between stars. So you can locate a handy bit of tossed out material, containing whatever you need (if basic elements or composites of such), pull up to it, harvest it, and process it. It's not something
we will be doing soon, but it is something on NASA's actual projections and planning out at the 50 to 75 years down the road... when we are first leaving this solar system.
I hesitate to go as far as you. Under the science-fantasy that is the GC universe and its framework, I do not see a reason that if they start off making their fuel in huge refineries (or labs) planet side, that they cannot refine and minaturize that over their advancing technology so that it is ultimately small enough to put onto a military ship as standard equipment. Perhaps early in the game, perhaps late. Whatever amuses Brad.
Also, higher tech doesn't necessarily need to utilize dense/heavy exotics. Indeed, most of our current material and power tech breakthroughs is coming out of simple and stable isotypes of carbon and silicon. If you have a few layers of twisted carbon nanotubes under your ship's skin, not only do you have an good insulator, but it could act as gigantic batteries. There are some simple blends of material that make excellent photocells. By using such a simple blend of material, you could be trapping some amount of the EM radiation that streams past your ship into those batteries. If you are just letting your ship idle, you could deploy huge sails of that blend of material to charge up your ship's batteries. So plenty of free power... if you aren't going anywhere or you are not changing direction for a long period of time. And since its made of trivially common material, if your power sails get torn, you just locate some free floating material (such as ejected asteriods, comets, planets), harvest what you need, and make more.
For high power demans (spike times), then perhaps the ship runs a deutrium fusion reactor. Fun thing with those basic isotypes is that even they are relatively common in our astronomical surveys. So there are sources for such, easy for FTL ships to reach. Note that. Not easy for us as we are stuck in the mud or toilet bowl racing, but for a race that can traverse the vast distances between stars in a few mere weeks, easy in reach for them.
That's the danger of using too much reality in coloring the GC framework. It isn't any more science based then Tolkien's LotR. It's just got a few labels that we see as belonging to a particular narrow bit of science. We can not truly conceive what tech near the transendance threshold would be like. Or what science has waiting for us behind actual transmutation of matter. That's the advantage a fantasy-history game like Civ has over a SF game... we have some knowledge of how we got to this point. But we don't know the path to take to get to what we want long term, technologically speaking. Which is why I keep saying: it's the fun of the game that matters.

Although talking about what could be, according to the tech/framework of GC can sometimes be almost as fun as playing the game.