Possible solutions:
1) Batteries. The Phantasm, which also uses the ambient power cells, uses disposable charges or batteries as "ammunition" for their energy weapons. Alternatively, maybe having a built-in battery as part of the railgun's shell to reduce energy consumption, and free up more power for positron turrets.
2) Magnetohydrodynamical generator (MHD generator). The Divine Comedy uses this system to use the waste heat from its laser systems to recycle it back into additional power. This can also be applied on the ship's railgun systems which would also generate lots of heat. This wouldn't "generate" power, but it would at least reduce the energy usage of the ship's weapons -- and boost the efficiency of any other reactors on board if you have them.
3) A fission reactor. While lower output than fusion/plasma, this method has a much lower heat signature (unlike fusion, there isn't a minimum temperature for reactions to occur), and a much smaller minimum size. They can also operate years or even decades without refueling, depending on how fast you burn up fuel.
If you're feeling particularly self-sufficient like a 'Spacer, you can even turn nuclear torpedo warheads into additional reactor fuel to boost energy output. Or inversely, use the reactor to "enrich" its own fuel to produce nuclear torpedo warheads, and depleted uranium for railgun shells. Self-sufficiency in both ammunition and power would allow you to operate very long periods of time without going back for resupply -- one of the most important aspect of commerce raiding.
4) Fusion (plasma?) reactors. These would give you limitless power, given what little energy-consuming equipment you have aboard now. The problem is they can be quite bulky, run "hot" only (therefore may produce a significant heat signature), and needs to dump spent fuel on occasion (which could leave a trail for pursuers, unless a friendly port is nearby).