The two engineers who need combat bonds (Ishmaak and Dekker) don't care where you got the combat bonds from - any system, anywhere in the Bubble will work. Just be aware, handing combat bonds in to an Engineer cancels any effect those bonds might have had on the BGS. So don't hand over any bonds to an engineer if you actually care about who wins the war.
But ordinarily, combat bonds can only be handed in to a space station where the faction that you fought for is present. The only other option is to go to a low security system and ask the Interstellar Factors mercenary contact to collect them for you (they take 25% of your profits if you choose this option). The advantage of it is, you can ask any interstellar factor to do this for you, even if they're on the opposite side of the Bubble, or out in Colonia, or whatever.