These are Fisher Pen refills modified for use in the Valiant Concepts keychain pen or the Illumn Micro Pen. They are pressurized and will work in almost any environment as they do not rely on gravity for the ink flow.
3 Pack - Schmidt 8127 Capless Rollerball Refill 4.31
Monfince Stainless Steel Pen Keychain with 5 refills, Pocket Pen
[VC Pen Titanium] Valiant Concepts Titanium Keychain Pen
Valiant Concepts Titanium Keychain Pen
Sold/Expired - Exotac Ti Water Bottle and Valiant Concepts
Pen Refills: Don't Throw Your Empty Pen Away—Refill It!
The NanoInk: Titanium Quick Release Eternity Pen by InnoZoom
Every Day Carry Gear (EDC)
Pen & Ink Pens, Lead Refills for sale
Pen Refills at Penn State Industries