if you know which one you need (IE the ring number) you can buy just that ring O-ring monkey, however like the previous guy said, i think O-ring monkey sell a Full O-ring set (EVERY O-RING YOUR MARKER TAKES) for about $15 for 10 x rebuilds, 1 x rebuilds for $7 and 5 x rebuild for $10, and sooner or later you will need the others, and it will cost you alot more buying them 1 at a time.