Few things here you should take into account

* If px-auto-complete-source is going to be subclassed the selector won't work. Probably want to run through the child and find the first one that implements the source
* You might want to have a mutation observer to pick up a change to the source or if the source is added programmatically.
* Might want to move the query to ready instead. http://www.polymer-project.org/docs/polymer/polymer.html#lifecyclemethods