The inside o-rings? Those are only necessary if you have a hard-tipped carto. If you have a soft-tipped cart, the soft tip acts as the drip protector like the o-ring.
Or do you mean the colored rubber band things some people use to mark their carts? IIRC, they're orthodontic rubber bands, and you should be able to
buy them online from any dental supply place or even from your own dentist.