Little.
I think it's 1/8" ID and 1/32" diameter ring. I also think it's hard to find -- I can never seem to find it looking in McMaster-Carr.
If you have a steady hand you can cut a chunk of small silicone fuel tubing about 1/32" long and make it work. The silicone will be squishy enough to absorb some irregularity in the cut, but not a whole lot -- which is why you need the steady hand.