meloniq

Enhance the default WordPress admin search by including custom fields (post meta) in search results. 
This plugin seamlessly extends the post listing (edit.php) search functionality, allowing administrators to quickly find content based on values stored in custom fields.
It works out of the box with no configuration required—just activate and start searching. 
Especially useful for sites that rely heavily on custom fields, such as those built with ACF or other meta-based data structures.
Lightweight, efficient, and fully integrated with the native WordPress admin experience.

WP-Admin Search Post Meta

Search WordPress admin posts by custom fields (post meta) directly from the default search.

Y.Kohno

This is a flexible search form widget. 
You can easily generate a multi condition search form and display anywhere as widget.
複数のカスタムフィールド、投稿タイプ、カテゴリー、タグでの検索が可能な複合検索フォームを簡単作成できるプラグイン。ウィジェットで好きな場所に設置可能。
Currently supports 
– Custom Fields 
– Taxonomies 
– Categories 
– Post tags 
– Post Type

WP Meta Search

Search form for searching through custom fields, post types, categories, tags, and free text.

WP-Admin Search Post Meta vs WP Meta Search