Gel Pen Refills 0.5 mm,100 Pack Ink Pen Refills Blue,0.5 mm Pen Refill Metal Tip,Pen Refills Size 130mm/5.1 inche,Send 2 Extra Pens,Smooth Writing.

Without seeing the refills that you have, one may assume that the Kuki pens use a very common .5mm refill like these: https://www.amazon.com/Refills-Replace-Universal-Refill-Writing/dp/B0C96K1LL9