This: http://extracatchy.net/wordpress-limit-search-results/

searchform.php

<form role="search" method="get" class="search-form" action="<?php echo home_url( '/' ); ?>">
  <div class="form-group">
    <div class="input-group">
      <input type="search" class="form-control form-input"
            placeholder="<?php echo esc_attr_x( 'Search for T-Shirts…', 'placeholder' ) ?>"
            value="<?php echo get_search_query() ?>" name="s"
            title="<?php echo esc_attr_x( 'Search for:', 'label' ) ?>" />
     <input type="hidden" name="post_type" value="t-shirts" /> <!-- Change value from "t-shirts" to whatever your custom post type name -->
      <div class="input-group-addon"><button class="form-btn" ><i class="fa fa-search" aria-hidden="true"></i></button></div>
    </div>
  </div>
</form>