Silicon

The primary source of silicon (especially early game) should be from extracting Sand or Loose Silt, either from a Desert planet or from an underground sub-biome on your starter world. On the off chance that you cannot acquire either of these resources for whatever reason, Silicon can also be purchased from Vindee's Gems at the Science Outpost as a last resort.

Later in the game you can build an automated hydroponic farm of Poe Trees, or smelt Silicon from common rocks such as Cobblestone or Meteorite Rock in a Blast Furnace.