I have an imperial O-ring in my water pipe from the water-pump. I went one thickness size larger, same diameter. it was a tight fit but it is working good...I just used lots of grease so it wouldn't roll off as I slid the water-pump onto the pipe.

Keep looking for a metric O-ring. I can guarantee you it isn't an odd "dealer item only" size. bearing shops carry all sorts of seals/o-rings, if they don't have the size you need they can definitely get it.