Yeah sand casting is the way to go, fast and for one piece. No sense spending the money on a proper mold, plus you can do it in aluminum.
Bronze would be good too. Bronze is tough stuff!
You could always plate it to match the other piece too, whatever metal it is in.
Also, have you tried the manufacturer? Perhaps they can send you the missing piece? (for a fee, no doubt) or it can be machined in aluminum (expensive) or done with lost wax and a plaster mold...
Many possibilities, depends on your budget of money and time.
Good luck!