Besides Scirra's website, does anyone know of another website where an indie game developer can get science fiction characters for free?
opengameart.org has quite a wide range of assets, not all of it is amazing though so it can take a while to find what you want.
I quite like Kenney assets, tend to be very high quality and they have a wide range of content. I'm not sure if they have anything that counts as "sci-fi" though. kenney.nl/assets
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To Nepeo:
Thank you for the information!
try itch.io, a lot of free assets on there.
To Frozen Well Studios:
Thank you, I will search for a game asset at this URL address.