This Omeka plugin allows to use Select2 JS library on every dropdown list in Omeka.

It is configurable by using CSS selectors. Select2 will be applied on every select that match the selectors. For instance, the default is:

body.item-types select.existing-element-drop-down,
body.advanced-search select.advanced-search-element

which enables Select2 on elements drop-down lists in advanced search and item type configuration pages