Ok so this is my understanding of how the system works with getting materials as a mission reward from my EXTENSIVE amount of grinding. So, as you may know, each planet has locked in set of minerals that you can find on that planet. Every planet has just 11 minerals and the minerals are persistent for that planet. Meaning it is the same for everyone. However, since only 11 elements are on each planet, some planets will not have Yttrium for example (You can find Yttrium on Deciant 7B btw). If someone says they found Yttrium on a planet, it WILL be there, but it is at the mercy of the RNGesus. Sooo I believe the same system works for the materials. Every space station has the common mats as a mission reward, but then the rare things IE Micro-Weave Cooling Hoses, will only be on certain stations. But then even on those stations, it will RNG the chances of it coming up. So you may very well get to a planet that has the possibility of having that as a reward, but you would need to STAY there and keep trying to beat out that damn RNG in order for it to come up. So, if someone says that found it on a specific station, it WILL be there again, you just have to stay there and keep trying. I could be wrong, but that seems to be how I've experienced it.