You may want to have a look at the video game FTL (Faster Than Light), a minimalistic spaceships combat game, where you scavenge wrecked ships to repair or improve your own and survive the next fights.
In this video game, the main currency is SCRAPS. They did not bother to tell which parts of the ship were taken, it's just a pile of metallic or electronic waste you can use to buy stuff, or directly to craft upgrades.
But you can also see there are other "currencies" :
- Fuel : fuel is quite a rare and valuable ressource need for long distance jumps.
- Ammunitions : some weapons require ammunitions, like missiles or bombs.
- Drone parts : this allows you to deploy little drones around your ships to perform various tasks.
If scraps are the only currency you can use "as is" on the market, like money, all these ressources can be traded at one point or another, making them a potential source of inspiration for your galactic currency.
Considering the hostility of, well, space environment, we may also imagine a few more :
- Food and water.
- Oxygen bottles.
- Medicin supplies.
Really, it could be anything valuable enough to be traded. The questions you should ask yourself are :
- How much worth do I wan't my currency to be ? (=> Do I spend 4 credits or 30000 to buy an new ship).
- Does my galactic society relies on bartering or does it need to be strictly considered a currency.
- How serious and plausible do I want my universe to be (=> There's quite a lot of issues, for examples, with a world that relies on bottle caps as a currency, but it didn't prevent Interplay and Black Isle Studio to invent a quite succesful one in 1997)