Here at fixya we don't sell any product or parts. We can advice you some online shop to find what you need. However in your case there in not any shop that selling any parts for the electric fan on stand not only for Holmes but any brand. The factorys they don't distribute any replacing parts to suppliers and they don't sell the direcly either.
So the only option you have is to mutch the screw cap with one of an broken fan that is mathing yours. You can even buy the simple moder with 16 inch and use the srew cup at link