Where can I get a replacement gasket?

Sodastream carbonating bottles use a silicone gasket in the bottle cap to seal in all the CO2 pressurized goodness. Unfortunately it gets mildew on it after a while and it's hard to get it out of the cap. There are those of us who are persistent about getting them out and getting them clean.

We tend to slice them up a bit over time and Sodastream doesn't sell replacements.

Anybody know where we could buy suitable drop-in replacements?

The gasket is nothing but a ring that appears to be die-cut out of a flat sheet of translucent silicone material.