www.sfarti.tk, some artifacts make you some money (only a little). Might be enough to get out of debt if you have some.
for the rest: planets, population and infra research. U get a small amount of money for plundering planets as well, but well, that takes 5 turns and you would loose money doing them if in debt.
You can allways restart if you in debt to deep or wish to get it over with quickly.