Where can I get synapse in Warframe?

Could you please clarify where exactly one can acquire synapse in the popular online game Warframe? I've been searching for it but haven't had much luck so far. Are there specific missions or quests that I should be focusing on? Or is there a specific vendor or location within the game where I can purchase it? I'm really interested in upgrading my weapons and armor, and I understand that synapse is a crucial resource for that. Any guidance you can provide would be greatly appreciated.