O-rings are generally measured by their cross-section diameter, and the inside diameter inside the "O" If you want to be cheap, you can just measure your O-ring (be careful not to squeeze excessively; we're talking undeformed diameters), and talk to an industrial/maintenance supply house. You could use the spare O-ring as the 'travel & storage' O-ring, and put on the regular O-ring the night before the dive.
There are several O-ring "standards" (sort of like screws, or soft drink bottle sizes...). I've heard that Canon's (and Oly's) use JIS, rather than ANSI (US) or metric sizing (JIS is actually metric, but let's not go there...). Once you know the specific O-ring parameters (which are cross-section, diameter, and Shore durometer, which is a measure of the 'rubber' hardness), you can get a replacement O-ring. Any industrial O-ring material will work quite well (at the right durometer) to replace the Canon O-ring. Common materials used are nitrile, viton, silicone, EPDM, & good old neoprene. They all have different chemical & temperature resistances, but for tropical/semitropical water diving, are all good.
