I seem to remember reading that those "O" rings weren't available separately from BMW.
They had to be sourced separately. They're not a weird cross-section like the solenoid "O" rings so shouldn't be a problem working out the diameter and thickness and purchasing from an auto parts store.
The guy at the BMW parts counter and I also couldn't find the ones that hold the solenoids in the solenoid board. Mine are loose and the solenoids tend to fall out easily when I'm cleaning them. I suppose they probably don't matter as much once the solenoid boards are installed. Either we just suck, or were looking at the wrong diagram. But since you can't buy individual solenoids from BMW, I wouldn't be surprised if they weren't available separately.
I saw a post recently saying these o-rings (which are not available from BMW) can be bought form Beisan Systems who specialize in VANOS stuff. I don't know if they come with a kit or can be bought separately.