Name of plastic piece
Just put it back in &amp; on, they are &amp; I call them bushings. They just center the smaller rod within the bigger rod. It's not a locking collar &amp; even if it were broken in 2 it would still function normally. You can sparingly glue it to the inside of the big rod if it keeps slipping out. Other than that, you should be able to find replacements at Home Improvement &amp; Fabric or Window Treatment stores.