Once you meet the unlock conditions for these, you can go into your Development menu of your iDroid and develop them provided that you meet the development requirements.

Item	How to unlock
Bandana	Complete Side Op 143.
F-Ballista (D-Walker)	Extract 1000 enemies.
Furicorn Uniform (D-Horse)	Complete Side Op 144.
Gray Fox (1998) Uniform	Complete all MIssions (excluding the High Difficulty Missions).
Gray XOF Uniform (Quiet)	Watch Quiet's Interrogation Scene.
Hand of Jehuty	Complete Side Op 49.
Infinity Bandana	Complete MIssion 46.
Leather Jacket Uniform	Complete Mission 46.
Naked Gold Uniform	Obtain an S-rank on all High Difficulty Missions.
Naked Silver Uniform	Complete all High Difficulty MIssions.
Raiden Uniform	Obtain an S-rank on all main Missions (excluding High Difficulty Missions).
Silver Bionic Arm	Complete all High Difficulty Missions.
Sniper Wolf Uniform (Quiet)	Complete MIssion 40.
Stealth Camo	Complete Huey Emmerich's story.
Tactical Fulton Unifrom (D-Dog)	Find the First Aid Manual Blueprint in Masa Village in Central Africa.
Water Pistol	Complete Mission 18.
Wormhole Fulton	Complete Side Op 50.