In the Research Tech Tree viewer, press "Q" to bring up "quick search." Type Defender. Select the armor topic, it will show you prereqs, and which you have, etc. You can click back through them to see your bottleneck, etc. In my experience, Defender armor is a late mid-game tech. This can take 2-3 years. You may have a lucky break and get "Military Transport" missions where Guild, Academy, Govt, Spartans, etc fly around in Gunships. The last Heavy Gunship in the wave often has some freebie sets of armor including Defender Armor. These can be a huge boost, but they are the luck of the draw (dependent on RNG to spawn the wave, dependent on your coverage of that area of the globe, and your ability to get there with a team that can take on strong enemy troops, etc.)