This pen uses the most popular refill-type in the world ...which is commonly known as the Parker-style refill (sometimes known as "G2").
Here's a list of some of the refills you can use in the pen:
Faber-Castell Ballpoint Pen Refill
Fisher Space Pen Refill, PR Series- Colors (Bold, dium, Fine)
Fisher Space Pen – Universal Ballpoint Refill
Monteverde Ceramic Gel Refill (Broad)
Monteverde Needle Point Refill (Fine)
Monteverde Soft Roll- Colored inks (Medium)
Monteverde Soft Roll- (Superbroad, Medium, Ultrafine)
OHTO Needlepoint Ballpoint Pen Refill PS-107NP
OHTO PS-205NP Extra-Fine 0.5mm Ballpoint Pen Refill
Parker Ballpoint Pen Refill (Broad, Medium, Fine)
Parker GEL Ballpoint Pen Refill (Medium)
Parker Quinkflow Ballpoint Pen Refill (Medium, Fine)
Pelikan Giant Ballpoint Pen Refill 337 (Broad, Fine, Medium)
Pentel KFLT8 Ballpoint Pen Refill
Tombow BR-ZLM Ballpoint Pen Refill
Visconti Ballpoint Pen Refill AA49 1.4 (Broad)
Visconti Gel Refill (Broad, Medium, Fine)
As long as your refill is in the style below - it will fit!