Just running through a few cases I've noticed where some costume parts from NPCs are available (Talons, Circle of Thorns, Longbow, Sky Raider porter pack) but others are not.
List of Examples:
- Wyvern Full Set and jetpack
- Sky Raider set (Helmet, gloves, jumpsuit, flight pack)
- Rulu-Shin Gloves, Chest Detail
- Sybil dress (belt currently in costume creator)
- Leonard/Dr. Aeon hex gloves, chestplate
- Resistance (Commando pants, Veteran chest detail/shoulders (?), Heavy Hands Gloves, Heavy barrel helmet, Officer dual pistols)
- Praetorian Police heavy gauntlets, heavy shoulders, pants, heavy vest, heavy boots/greaves
- Knives of Vengeance tactical goggles (unable to find a good screenshot)
- Malta Sappers use Zero-G pack as an always on backpack, similar to Sky Raiders
I'm sure there's more out there, these were just the first to come to mind.