I ran into something similar. I'd removed a hose and found that it didn't have that hard ring...and the o-ring, though it had sealed, was stretched out severely.
It may be that you might still get it it and have the hard ring hold it properly in place...and just make sure your missing pieces are from the right part of the hose...and frankly, make sure that there isn't already an o-ring or other ring in there...sometimes people do stupid things (yes, I mean person before you, not you!).
If the hard ring fits in but the o-ring doesn't, use the o-ring from the other end of the hose (assuming you still have that intact...to spec the size o-ring you need...and I know that the dealer doesn't sell these alone...at least not for the hose...and I doubt there's another 0-ring that bmw sells that would fit.
Oh, one final thought...there may be a groove inside the fitting that you missed and if so, the o-ring might not be as bad a fit as you think...and you have already tried adding the spacer after it to see if it holds it in place?
Just make sure when you put it back together, that you add that extra ring in the correct orientation...match the other end of hose.
BTW, if you do find a good replacement from hardware store, would be nice 'for the record' to know what size that is...make sure it's thick enough to protrude above the other ring...and good luck!
Yea, btw, the hard ring won't do anything for sealing...it's just for spacing the fittings together properly...but you figured that out already...should've asked, could've saved you some time. It's a pressure fitting, so need o-ring!
Doug