Had the same issue, you won't be able to buy a ring by itself farom anywhere, options are second hand shaft (rare) new from mb (£700 from memory) or get yours spot welded back on like I did.
If it has split because of corrosion between the ring and shaft bursting it you can clean both faces with emery cloth then Araldite it back in place, clamping the ring on with a Jubilee clip while the glue goes off. It's a bodge, but one that works long term and is about £696.75 cheaper than a new shaft!
