Defiant gear was introduced some time during Secrets of Faydwer. It's gear that drops all around the world from mobs of every type. The higher level the mob, the higher level the Defiant gear it will drop. Due to how common Defiant gear is to find it doesn't sell that well in the Bazaar.
Below is a list of all the tiers of defiant gear.
Crude Defiant Gear (Recommended level of 10. No required level.)
Q: What expansion do I need to get this gear?
A: None, you can get it and equip it with any expansion.
Q: How expensive is this gear in the bazaar?
A: Usually less then 100pp for the weapons and 250-2500pp for each piece of gear.
Q: How common is this gear?
A: By level 50 you'll probably see two dozen pieces drop.