Go to your dealer only pennies part no 51248230139 or here http://www.bmminiparts.com/PartDetails.aspx?InventoryId=69ACB72F-04B1-4F7D-BA64-CC5BC73D1C8E
Having just attached said 'stoppers" ...
Save yourself time and money by using a piece of 1/2 inch rubber tube...
Cut two sections about 3/4 of an inch. slit them lengthwise if you wish, and attach with a suitable glue.
The actual contact point on the 'stoppers is minimal.
Pretty much what I did too. I had some clear plastic tube lying about that fitted tightly on the hinge part, but that wasn't quite thick enough to give enough clearance. The plastic tube fitted tightly inside some half inch hose pipe so that went on to finish it.