I have a Red Sea regulator and asked Red Sea how much a new seal will cost. They are about $3.50 AUD for a Red Sea one, but I notice that home brew shops also sell CO2 regulator seals for less than $1.
Go to any Gas cylinder refill place, they typically will give some for free.
Ask when you get the tank refilled, they have to be standard sizes, otherwise you'd have to use their special gas tanks and regs, and it would very hard to find someone to refill their special CO2 tanks.
I found a 'CO2 washer' on a website with the inner/outer diameter, outer diameter was about 19mm, inned diameter (i.e. diameter of the central hole) was only about 5-6mm. The inner diameter of my CO2 seal is 10mm. Is it possible that Red Sea have made their regulators so that only one of their CO2 seals will fit? ;-)
I have the same problem with my Tunze regulator. Standard replacement washers don't fit it for the same reason you seem to be talking about. I just use a rubber washer instead. Found it at Ace Hardware I think and I doubt it's meant for CO2. But it fits—and it works.