<div data-hook="admin_orders_index_search">
  <%= form_for :variant_search, url: form_path, method: :get do |f| %>
    <div class="row">
      <div class="field-block col-3">
        <div class="field">
          <%= label_tag nil, Spree::StockLocation.model_name.human %>
          <%= select_tag :stock_location_id, options_from_collection_for_select(stock_locations, :id, :name, params[:stock_location_id]), { include_blank: true, class: 'select2 fullwidth', "data-placeholder" => Spree.t(:select_a_stock_location) } %>
        </div>
      </div>
      <div class="<%= if content_for?(:sidebar) then 'col-6' else 'col-9' end %>">
        <div data-hook="add_product_name" class="field">
          <%= label_tag :add_variant_id, Spree::Variant.model_name.human %>
          <%= text_field_tag :variant_search_term, params[:variant_search_term], class: "fullwidth", placeholder: Spree.t(:variant_search_placeholder) %>
        </div>
      </div>
    </div>

    <div class="clearfix"></div>

    <div class="actions filter-actions">
      <div data-hook="admin_orders_index_search_buttons">
        <%= button Spree.t(:filter_results) %>
      </div>
    </div>
  <% end %>
</div>

<div data-hook="admin_orders_index_search">
  <%= form_for :variant_search, url: form_path, method: :get do |f| %>
    <div class="row">
      <div class="field-block col-3">
        <div class="field">
          <%= label_tag nil, Spree::StockLocation.model_name.human %>
          <%= select_tag :stock_location_id, options_from_collection_for_select(stock_locations, :id, :name, params[:stock_location_id]), { include_blank: true, class: 'custom-select fullwidth', "data-placeholder" => Spree.t(:select_a_stock_location) } %>
        </div>
      </div>
      <div class="<%= if content_for?(:sidebar) then 'col-6' else 'col-9' end %>">
        <div data-hook="add_product_name" class="field">
          <%= label_tag :add_variant_id, Spree::Variant.model_name.human %>
          <%= text_field_tag :variant_search_term, params[:variant_search_term], class: "fullwidth", placeholder: Spree.t(:variant_search_placeholder) %>
        </div>
      </div>
    </div>

    <div class="clearfix"></div>

    <div class="actions filter-actions">
      <div data-hook="admin_orders_index_search_buttons">
        <%= button Spree.t(:filter_results) %>
      </div>
    </div>
  <% end %>
</div>

Use custom-select for variant search
Use custom-select for variant search
