DeepColdWaterAccording to the Well Appointed Desk, any of the following refills work for a pen that uses the G2 refill:
G2/Schmidt/Euro Rollerball (and similar) Refills:
The Pilot G2 style refill, also described as the Schmidt/Euro Rollerball refill size are about 110mm long:
Caran d’Ache Fibre Tip Refill
Caran d’Ache Rollerball Refill
Faber-Castell Rollerball Pen Refill
Faber-Castell Fineliner Pen Refill
FORAY (Office Depot) Pen Refill For Schmidt Rollerball
Kaweco “soul” refill G2
Kaweco G2 Gel Roller Pen Refill
Hi-Tec-C Cavalier refill
Monteverde Fineliner Refill G52
Monteverde Ceramic Rollerball Refill W22
Monteverde Spring-Loaded Tip Fineliner Refill G62
OHTO Ceramic Rollerball Pen Refill
OHTO F-300 Fude Refill Black
Pentel LR7 Energel
Pilot Frixion Erasable Gel
Pilot Juice (removed from plastic retractable pen)
Pilot G2
Pilot LP2RF Gel Refill (Juice refill code)
Schmidt Capless Rollerball 8126 and 8127 (shorter models can work with an extension hack)
Schmidt FineLiner refill
Schmidt Safety Rollingball Ceramic refill
Schmidt Safety Rolling Tube refill
Schmidt RollingSupersteel refill
Schmidt RollingBall refill
Schneider Topball 850 Pen Refill
Schneider Topliner 970 Pen Refill
Sheaffer Fineliner Refill
Visconti Rollerball Pen Refill AA40 0.7
Zebra JT refill
Zebra Sarasa Clip refill (removed from plastic retractable pen)
http://www.wellappointeddesk.com/2014/06/the-epic-refill-reference-guide-rollerball-gel-and-ballpoints/