The easiest way to farm raw materials is to go to the planets listed below, they are all about 1500 light years out so bring a fuel scoop. These have crystalline structures where all crystalline clusters will drop a single, grade 5 mat. You fly down to the surface and you will see the structures, they look like pointy objects, these are what you are looking for, shoot the clusters off and scoop them.
The darker blue color is what you will look for and the structures seem to be prevalent in the hilly or mountainous areas although on HIP 36601 Planet C1A I found them on the open plains. Because it's in a mountainous area, I would recommend that you use a small or medium ship because it can be hard to find a landing spot at times. You can only see them from like 100 meters up, going slow, night vision helps, even in daylight. When you land, you will see a double band, high up on the scanner screen, these are what you are looking for.
I haven't been to all the planets, missing the last couple of them. Before you go, visit a material trader and trade down all grade 5 mats. When you are done, you will have a full suite of grade 5 mats that you can trade down. Enjoy!
Goto HIP 36601
Planet C1A All crystalline clusters will be Polonium.
Planet C1D - All crystalline clusters will be Ruthenium.
Planet C3B - All crystalline clusters will be Tellurium.
Planet C5A - All crystalline clusters will be Technetium.
Goto Outotz LS-K D8-3
Planet B5A - All crystalline clusters will be Yttrium.
Planet B5C - All crystalline clusters will be Antimony.
Goto LHS 417
Planet 9EA - All clusters will be Selenium.