Your question is a little unclear. Are you asking what you can harvest from space to use as an energy source for a spacecraft? If so, then where in space are you asking about? Interplanetary space? Interstellar space? Nebulae? Do you want to know what you can harvest from gas giants, asteroids, comets, etc?
It's pretty thinly spread out in interstellar space though, not enough there be able to collect it very effectively.
Even if there was enough, in order to use it as a power source you would need some kind of very efficient but light weight fusion technology,
certainly way beyond any technology we can presently foresee.
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