typically...
1) the colder a planet is (each planet has random temp roll, 15 is the coldest, 1 is the hotest) the more deut you can get, at the cost of less efficient solar sats.
2) raid it from the area.
3) trade with some of the large players, often the amount you need is less significant to them, so if you're on good terms they will trade with you as long as the ratios are right
4) crash a stockpile.
alternatively ruby it in. (merchants or imperial support)